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mid_content, all: [300,250][336,280][728,90][970,250][970,90][320,50][468,60]--> Everything With a "Value" is True Examples 100 3.14 -15 "Hello" "false" 7 + 1 + 3.14 Try it Yourself » Everything Without a "Value" is False The Boolean value of 0 (zero) is false:function a(){ { // this is the Max Scope for let variable let x = 12; } console.log(x); } a(); // Uncaught ReferenceError: x is not defined When Using varfunction run() { console.log(foo); // undefined var foo = "Foo"; console.log(foo); // Foo } run(); let variables are not initialized until their definition is evaluated. Accessing them before the initialization results in a ReferenceError. Variable said to be in "temporal dead zone" from the start of the block until the initialization is processed.MDN Web DocsWeb TechnologiesLearn Web DevelopmentAbout MDNFeedbackAboutContact UsFirefoxMDNMozilla© 2005-2020 Mozilla and individual contributors. Content is available under these licenses.

const a = 42; a = 43; // TypeError: Assignment to constant variable. Note that it doesn't mean that the value is immutable. Its properties still can be changed.// prints out 'undefined' console.log(typeof undeclaredVariable); // results in a 'ReferenceError' console.log(typeof i); let i = 10; Another example of temporal dead zone combined with lexical scoping Due to lexical scoping, the identifier foo inside the expression (foo + 55) evaluates to the if block's foo, and not the overlying variable foo with the value of 33.discountPrices([100, 200, 300], .5) // [50, 100, 150] And the implementation might look something like thisconst obj = {}; obj.a = 42; console.log(obj.a); // 42 If you want to have an immutable object, you should use Object.freeze().So between variables that change and variables that don’t change, there’s not much left. That means you shouldn’t ever have to use var again.

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let - JavaScript MD

let x = 1; let x = 2; // SyntaxError: Identifier 'x' has already been declared 4. Globals not attached to window: There are many ways JavaScript provides us with the ability to make it behave like an asynchronous language. One of them is with the Async-Await clause one of the most famous interview questions on scope also can suffice the exact use of let and var as below;

1. No use outside nearest enclosing block: This block of code will throw a reference error because the second use of x occurs outside of the block where it is declared with let: Get to know JavaScript Promises better. Understanding Promises. A Promise in short: Imagine you are a kid.Your mom promises you that she'll get you a new phone next week.. You don't know if you will get that phone until next week. Your mom can either really buy you a brand new phone, or she doesn't, because she is not happy :(.. That is a promise.A promise has 3 states

javascript - What's the difference between using let and

Also, consider that block scoped variables are not known before they are declared because they are not hoisted. You're also not allowed to redeclare the same block scoped variable within the same block. This makes block scoped variables less error prone than globally or functionally scoped variables, which are hoisted and which do not produce any errors in case of multiple declarations. Difference between var and let in JavaScript. var and let are both used for variable declaration in javascript but the difference between them is that var is function scoped and let is block scoped. It can be said that a variable declared with var is defined throughout the program as compared to let Syntax let var1 [= value1] [, var2 [= value2]] [ varN [= valueN]; Parameters var1, var2, , varN The names of the variable or variables to declare. Each must be a legal JavaScript identifier. value1, value2, , valueN Optional For each variable declared, you may optionally specify its initial value to any legal JavaScript expression function go(n) { // n here is defined! console.log(n); // Object {a: [1,2,3]} for (let n of n.a) { // ReferenceError console.log(n); } } go({a: [1, 2, 3]}); Other situations When used inside a block, let limits the variable's scope to that block. Note the difference between var, whose scope is inside the function where it is declared. <button ="alert('Hello')">Click me.</button> Submit Answer » Start the Exercise

let statistics = [16, 170, 10]; let [age, height, grade] = statistics; console.log(height) 2.var may still be useful if you are sure you want the single-binding effect when using a closure in a loop (#5) or for declaring externally-visible global variables in your code (#4). Use of var for exports may be supplanted if export migrates out of transpiler space and into the core language.In the example below, we create a variable called carName and assign the value "Volvo" to it.In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input.

var globalVariable = 42; let blockScopedVariable = 43; console.log(globalVariable); // 42 console.log(blockScopedVariable); // 43 console.log(this.globalVariable); // 42 console.log(this.blockScopedVariable); // undefined Inside a function Inside a function (but outside of a block), let has the same scope as var.const height = 400; const width = 200; var newParagraph = document.createElement("p"); var newText = document.createTextNode("Height is : "+height+" Width is : "+ width +". So, Area of the Rectangle is " + height*width + " sq.ft."); newParagraph.appendChild(newText); document.body.appendChild(newParagraph); View the example in the browser "camel-case" "some-text" "some-mixed-string-with-spaces-underscores-and-hyphens" "all-the-small-things" "i-am-listening-to-fm-while-loading-different-url-on-my-browser-and-also-editing-some-xml-and-html"  function testVar () { if(true) { var foo = 'foo'; } console.log(foo); } testVar(); // logs 'foo' function testLet () { if(true) { let bar = 'bar'; } console.log(bar); } testLet(); // reference error // bar is scoped to the block of the if statement variables with var:

var vs let vs const in JavaScript - TylerMcGinnis

var a = 1; var b = 2; if (a === 1) { var a = 11; // the scope is global let b = 22; // the scope is inside the if-block console.log(a); // 11 console.log(b); // 22 } console.log(a); // 11 console.log(b); // 2 Also don't forget it's ECMA6 feature, so it's not fully supported yet, so it's better always transpiles it to ECMA5 using Babel etc... for more info about visit babel website JavaScript lets you execute code when events are detected. HTML allows event handler attributes, with JavaScript code, to be added to HTML elements. With single quotes: <element event='some JavaScript'> With double quotes: <element event=some JavaScript> In the following example, an onclick attribute (with code), is added to a <button> element Loading… Log in Sign up current community Stack Overflow help chat Meta Stack Overflow your communities Sign up or log in to customize your list. more stack exchange communities company blog By using our site, you acknowledge that you have read and understand our Cookie Policy, Privacy Policy, and our Terms of Service. // An array of adder functions. var adderFunctions = []; for (var i = 0; i < 1000; i++) { // We want the function at index i to add the index to its argument. adderFunctions[i] = function(x) { // What is i bound to here? return x + i; }; } var add12 = adderFunctions[12]; // Uh oh. The function is bound to i in the outer scope, which is currently 1000. console.log(add12(8) === 20); // => false console.log(add12(8) === 1008); // => true console.log(i); // => 1000 // It gets worse. i = -8; console.log(add12(8) === 0); // => true The process above doesn't generate the desired array of functions because i's scope extends beyond the iteration of the for block in which each function was created. Instead, at the end of the loop, the i in each function's closure refers to i's value at the end of the loop (1000) for every anonymous function in adderFunctions. This isn't what we wanted at all: we now have an array of 1000 different functions in memory with exactly the same behavior. And if we subsequently update the value of i, the mutation will affect all the adderFunctions.

JavaScript Variables - W3School

if (x) { let foo; let foo; // SyntaxError thrown. } You may encounter errors in switch statements because there is only one block.The variable carName will still have the value "Volvo" after the execution of these statements: 7 If I read the specs right then let thankfully can also be leveraged to avoid self invoking functions used to simulate private only members - a popular design pattern that decreases code readability, complicates debugging, that adds no real code protection or other benefit - except maybe satisfying someone's desire for semantics, so stop using it. /rant{ x = x + 1; // ReferenceError during parsing: "x is not defined". let x; console.log(`x is ${x}`); // Never runs. } In contrast, the same example with var parses and runs without throwing any exceptions.3. No redeclaration: The following code demonstrates that a variable declared with let may not be redeclared later:

As you can see the var j variable is still having a value outside of the for loop scope (Block Scope), but the let i variable is undefined outside of the for loop scope. { let l = 'let'; const c = 'const'; var v = 'var'; v2 = 'var 2'; } console.log(v, this.v); console.log(v2, this.v2); console.log(l); // ReferenceError: l is not defined console.log(c); // ReferenceError: c is not defined function discountPrices (prices, discount) { var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } return discounted } Seems simple enough but what does this have to do with block scope? Take a look at that for loop. Are the variables declared inside of it accessible outside of it? Turns out, they are.console.log(noTDZ); // undefined var noTDZ = 43; console.log(hasTDZ); // ReferenceError: hasTDZ is not defined let hasTDZ = 42; No re-declaring You can't declare the same variable multiple times using let. You also can't declare a variable using let with the same identifier as another variable which was declared using var. Now, let me show you how they differ. Global window object. Even if the let variable is defined as same as var variable globally, the let variable will not be added to the global window object.. See the example below - var varVariable = this is a var variable; let letVariable = this is a let variable; Say, here we have two variables declared. let us see what output it actually.

A JavaScript function is a block of code designed to perform a particular task. A JavaScript function is executed when something invokes it (calls it). A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses (). Function names can contain letters, digits, underscores, and dollar signs (same. However, you cannot reference the variable declared with let or const until it is declared. If you try to use it before it is declared, then an exception will be raised during the Execution Phase of the Execution Context. Now the variable will still be in memory, courtesy of the Creation Phase of the Execution Context, but the Engine will not allow you to use it:Do not create Boolean objects. It slows down execution speed.The new keyword complicates the code. This can produce some unexpected results:

JavaScript Booleans - W3School

  1. 3 let is a part of es6. These functions will explain the difference in easy way.
  2. 4 Some hacks with let:
  3. Yaphi and James walk you through making your own simple JavaScript quiz. This is a fun exercise and a great way to learn a variety of coding techniques
  4. 2 As mentioned above:
  5. In the last post we learned all about how scoping works with JavaScript let, const and var variables. We now know that var is **function scope**, and now we know that let and const are block scope, which means any time you've got a set of curly brackets you have block scope. Now, we need to know you can only declare a variable inside of its scope once
  6. add a comment  |  2 I want to link these keywords to the Execution Context, because the Execution Context is important in all of this. The Execution Context has two phases: a Creation Phase and Execution Phase. In addition, each Execution Context has a Variable Environment and Outer Environment (its Lexical Environment).

JavaScript Functions - W3School

Cool, so anytime you want a variable to be immutable, you can declare it with const. Well, not quite. Just because a variable is declared with const doesn’t mean it’s immutable, all it means is the value can’t be re-assigned. Here’s a good example.- Supported Firefox & Chrome (V8). - In the case of Safari 5.1.7 and Opera 12.00, you can change the value of the const after defining a variable with const. - It is not supported in Internet Explorer 6-9, or in the preview of Internet Explorer 10.

193 What's the difference between let and var? A variable defined using a var statement is known throughout the function it is defined in, from the start of the function. (*) A variable defined using a let statement is only known in the block it is defined in, from the moment it is defined onward. (**) To understand the difference, consider the following code:If you want to report an error, or if you want to make a suggestion, do not hesitate to send us an e-mail: A Gentle Introduction to ES6. You must understand var to grasp the benefits of let / const.Let's rewind. Review: Variable Declarations. It's important to intentionally declare your variables within a specific scope, using var, to keep your code clear and maintainable Whereas hfoo, defined by var is initially in the global scope, but again when we declare it inside the if clause, it considers the global scope hfoo, although var has been used again to declare it. And when we re-assign its value we see that the global scope hfoo is also affected. This is the primary difference.I've heard it that it's described as a "local" variable, but I'm still not quite sure how it behaves differently than the var keyword.

JavaScript Events - W3School

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2 I think the terms and most of the examples are a bit overwhelming, The main issue i had personally with the difference is understanding what a "Block" is. At some point i realized, a block would be any curly brackets except for IF statement. an opening bracket { of a function or loop will define a new block, anything defined with let within it, will not be available after the closing bracket } of the same thing (function or loop); With that in mind, it was easier to understand:sidebar_sticky, desktop: [120,600][160,600][300,600][300,250]--> REPORT ERROR PRINT PAGE FORUM ABOUT × Report Error If you want to report an error, or if you want to make a suggestion, do not hesitate to send us an e-mail:Just like const the let does not create properties of the window object when declared globally (in the top-most scope).

JavaScript : const Statement - w3resourc

  1. function discountPrices (prices, discount) { var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } Remember that we were able to log i, discountedPrice, and finalPrice outside of the for loop since they were declared with var and var is function scoped. But now, what happens if we change those var declarations to use let and try to run it?
  2. A general workaround is to wrap this in an anonymous function and pass i as an argument. Such issues can also be avoided now by using let instead var as shown in the code below.
  3. function process(data) { //... } var hugeData = { .. }; process(hugeData); var btn = document.getElementById("mybutton"); btn.addEventListener( "click", function click(evt){ //.... }); The click handler callback does not need the hugeData variable at all. Theoretically, after process(..) runs, the huge data structure hugeData could be garbage collected. However, it's possible that some JS engine will still have to keep this huge structure, since the click function has a closure over the entire scope.
  4. const a; // SyntaxError: Missing initializer in const declaration share | improve this answer | follow | | | | edited Oct 25 '18 at 20:54 ketchupisred 61133 silver badges1616 bronze badges answered Nov 23 '16 at 22:52 Michał PerłakowskiMichał Perłakowski 59k2121 gold badges127127 silver badges138138 bronze badges add a comment  |  51 Here is an example for the difference between the two (support just started for chrome):
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  6. let x = 1; switch(x) { case 0: let foo; break; case 1: let foo; // SyntaxError for redeclaration. break; } However, it's important to point out that a block nested inside a case clause will create a new block scoped lexical environment, which will not produce the redeclaration errors shown above.

function varTest() { var x = 1; if (true) { var x = 2; // same variable! console.log(x); // 2 } console.log(x); // 2 } function letTest() { let x = 1; if (true) { let x = 2; // different variable console.log(x); // 2 } console.log(x); // 1 } share | improve this answer | follow | | | | answered Dec 17 '17 at 10:47 vipul jainvipul jain 7444 bronze badges add a comment  |  3 The below shows how 'let' and 'var' are different in the scope:Functions can be used the same way as you use variables, in all types of formulas, assignments, and calculations.

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When using the === operator, equal booleans are not equal, because the === operator expects equality in both type and value.let will not hoist to the entire scope of the block they appear in. By contrast, var could hoist as below.{ var globalVariable = 42; let blockScopedVariable = 43; console.log(globalVariable); // 42 console.log(blockScopedVariable); // 43 } console.log(globalVariable); // 42 console.log(blockScopedVariable); // ReferenceError: blockScopedVariable is not defined Inside a loop Variables declared with let in loops can be referenced only inside that loop. Not really a performance bottleneck, so much as saving time not having to type 'let' more than a handful of times. And I know it's basic javascript variable declaration, but was unaware if the syntax was the same in react native / jsx, and googling really didn't turn up much

Arrow function expressions - JavaScript MD

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24 Variable Not Hoistingvar SomeConstructor; { let privateScope = {}; SomeConstructor = function SomeConstructor () { this.someProperty = "foo"; privateScope.hiddenProperty = "bar"; } SomeConstructor.prototype.showPublic = function () { console.log(this.someProperty); // foo } SomeConstructor.prototype.showPrivate = function () { console.log(privateScope.hiddenProperty); // bar } } var myInstance = new SomeConstructor(); myInstance.showPublic(); myInstance.showPrivate(); console.log(privateScope.hiddenProperty); // error See 'Emulating private interfaces' JavaScript Data Types. JavaScript variables can hold numbers like 100 and text values like John Doe. In programming, text values are called text strings. JavaScript can handle many types of data, but for now, just think of numbers and strings. Strings are written inside double or single quotes. Numbers are written without quotes varname1, varname2......varnameN : Constant names. value1, value2......value3 : Value of the constant.

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Difference between var and let in JavaScript - GeeksforGeek

Let's step away from the individual data structures and talk about the iterations over them. In the previous chapter we saw methods map.keys (), map.values (), map.entries (). These methods are generic, there is a common agreement to use them for data structures. If we ever create a data structure of our own, we should implement them too // print '5' 5 times for (var i = 0; i < 5; ++i) { setTimeout(function () { console.log(i); }, 1000); } However, replace var with let The export statement is used when creating JavaScript modules to export live bindings to functions, objects, or primitive values from the module so they can be used by other programs with the import statement. Bindings that are exported can still be modified locally; when imported, although they can only be read by the importing module the value updates whenever it is updated by the exporting. var x = 'global'; let y = 'global'; console.log(this.x); // "global" console.log(this.y); // undefined When used inside a block, let limits the variable's scope to that block. Note the difference between var whose scope is inside the function where it is declared.

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let in the loop can re-binds it to each iteration of the loop, making sure to re-assign it the value from the end of the previous loop iteration. Consider,Variables declared by let have as their scope the block in which they are defined, as well as in any contained sub-blocks. In this way, let works very much like var. The main difference is that the scope of a var variable is the entire enclosing function:

var cat = "cat"; let dog = "dog"; var animals = () => { var giraffe = "giraffe"; let lion = "lion"; console.log(cat); //will print 'cat'. console.log(dog); //will print 'dog', because dog was declared outside this function (like var cat). console.log(giraffe); //will print 'giraffe'. console.log(lion); //will print 'lion', as lion is within scope. } console.log(giraffe); //will print 'giraffe', as giraffe is a global variable (var). console.log(lion); //will print UNDEFINED, as lion is a 'let' variable and is now out of scope. share | improve this answer | follow | | | | answered Apr 18 '19 at 0:49 daCodadaCoda 1,88833 gold badges1919 silver badges2525 bronze badges add a comment  |  4 ES6 introduced two new keyword(let and const) alternate to var.2. No use before declaration: This block of code will throw a ReferenceError before the code can be run because x is used before it is declared:

letとvarの違い - Qiit

  1. ated.
  2. And, finally, when it comes to the Execution Context, variables defined with var will be attached to the 'this' object. In the global Execution Context, that will be the window object in browsers. This is not the case for let or const.
  3. var x = 'global'; let y = 'global'; console.log(this.x); // "global" console.log(this.y); // undefined Emulating private members In dealing with constructors it is possible to use the let bindings to share one or more private members without using closures:
  4. During the Creation Phase of an Execution Context, var, let and const will still store its variable in memory with an undefined value in the Variable Environment of the given Execution Context. The difference is in the Execution Phase. If you use reference a variable defined with var before it is assigned a value, it will just be undefined. No exception will be raised.
  5. Every single click handler will refer to the same object because there’s only one counter object which holds 6 so you get six on each click.
  6. // Let's try this again. // NOTE: We're using another ES6 keyword, const, for values that won't // be reassigned. const and let have similar scoping behavior. const adderFunctions = []; for (let i = 0; i < 1000; i++) { // NOTE: We're using the newer arrow function syntax this time, but // using the "function(x) { ..." syntax from the previous example // here would not change the behavior shown. adderFunctions[i] = x => x + i; } const add12 = adderFunctions[12]; // Yay! The behavior is as expected. console.log(add12(8) === 20); // => true // i's scope doesn't extend outside the for loop. console.log(i); // => ReferenceError: i is not defined This time, i is rebound on each iteration of the for loop. Each function now keeps the value of i at the time of the function's creation, and adderFunctions behaves as expected.

JavaScript Let Previous Next W3Schools is optimized for learning, testing, and training. Examples might be simplified to improve reading and basic understanding. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content function process(data) { //... } { // anything declared inside this block can be garbage collected let hugeData = { .. }; process(hugeData); } var btn = document.getElementById("mybutton"); btn.addEventListener( "click", function click(evt){ //.... }); let loops A JavaScript Boolean represents one of two values: true or false. Very often, in programming, you will need a data type that can only have one of two values, like. For this, JavaScript has a Boolean data type. It can only take the values true or false. The Boolean () Function. You can use the Boolean () function to find out if an expression (or. Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).

JavaScript ES6+: var, let, or const? - Mediu

  1. for (let i = 0; i < 10 ; i++) { setTimeout( function a() { console.log(i); //print 0 to 9, that is literally AWW!!! }, 100 * i); } This is because when using let, for every loop iteration the variable is scoped and has its own copy.
  2. When the second function testLet gets called the variable bar, declared with let, is only accessible inside the if statement. Because variables declared with let are block scoped (where a block is the code between curly brackets e.g if{} , for{}, function{}).
  3. function discountPrices (prices, discount) { var discounted = undefined var i = undefined var discountedPrice = undefined var finalPrice = undefined discounted = [] for (var i = 0; i < prices.length; i++) { discountedPrice = prices[i] * (1 - discount) finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } Notice all the variable declarations were assigned a default value of undefined. That’s why if you try access one of those variables before it was actually declared, you’ll just get undefined.
  4. While variables declared with var keyword are hoisted (initialized with undefined before the code is run) which means they are accessible in their enclosing scope even before they are declared:

JavaScript Promises: An introduction - Google Developer

  1. Perhaps the most important thing you can learn to be a better coder is to keep things simple. In the context of identifiers, that means that a single identifier should only be used to represent
  2. for (var i = 0; i < 5; i++) { setTimeout(_ => console.log(`i is ${i}`), 125/*ms*/); } ... the loop unexpectedly outputs "i is 5" five times:
  3. NOTE: This is a teaching example intended to demonstrate the var/let behavior in loops and with function closures that would also be easy to understand. This would be a terrible way to add numbers. But the general technique of capturing data in anonymous function closures might be encountered in the real world in other contexts. YMMV.
  4. Example2: In my second example instead of using var keyword in my conditional block I declare myvar using let keyword . Now when I call myfunc I get two different outputs: myvar=20 and myvar=10.
  5. ES2015 introduced two important new JavaScript keywords: let and const. Variables defined with const behave like let variables, except they cannot be reassigned: Declaring a variable with const is similar to let when it comes to Block Scope. The x declared in the block, in this example, is not the same as the x declared outside the block
  6. Using the example above, toCelsius refers to the function object, and toCelsius() refers to the function result.

let works very much like var. The main difference is that the scope of a var variable is the entire enclosing function// print 1, 2, 3, 4, 5. now for (let i = 0; i < 5; ++i) { setTimeout(function () { console.log(i); }, 1000); } Because let create a new lexical environment with those names for a) the initialiser expression b) each iteration (previosly to evaluating the increment expression), more details are here.

var, let, or const? Hacker Noo

What that means is if you create a variable with var, that variable is “scoped” to the function it was created in and is only accessible inside of that function or, any nested functions. JavaScript is a scripting language that declares its variables in different types using keywords such as let, const or var. let and const have the same characteristics in terms of lexical scope. Lexical scope is defined as the visibility of the variables' values across the defined or enclosed block which holds the valid values and can be. 8 When Using let

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function varTest() { var x = 31; if (true) { var x = 71; // Same variable! console.log(x); // 71 } console.log(x); // 71 } function letTest() { let x = 31; if (true) { let x = 71; // Different variable console.log(x); // 71 } console.log(x); // 31 } share | improve this answer | follow | | | | edited Nov 26 '16 at 16:18 Peter Mortensen 25.7k2121 gold badges9090 silver badges118118 bronze badges answered Dec 17 '15 at 3:22 Abdennour TOUMIAbdennour TOUMI 52.4k2424 gold badges177177 silver badges186186 bronze badges add a comment  |  13 let is interesting, because it allows us to do something like this:function a(){ // this is the Max Scope for var variable { var x = 12; } console.log(x); } a(); // 12 If you want to know more continue reading belowfunction discountPrices (prices, discount) { console.log(discounted) // undefined var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } I can’t think of any use case where you’d actually want to access a variable before it was declared. It seems like throwing a ReferenceError would be a better default than returning undefined. In fact, this is exactly what let does. If you try to access a variable declared with let before it’s declared, instead of getting undefined (like with those variables declared with var), you’ll get a ReferenceError. 1 purpose? it's a construct, you can use it however you please, one of the interesting ways is like this. – Dmitry Nov 24 '16 at 0:39 add a comment  |  13 Function VS block scope: The main difference between var and let is that variables declared with var are function scoped. Whereas functions declared with let are block scoped. For example:Now the most important question we haven’t answered yet, should you use var, let, or const? The most popular opinion, and the opinion that I subscribe to, is that you should always use const unless you know the variable is going to change. The reason for this is by using const, you’re signalling to your future self as well as any other future developers that have to read your code that this variable shouldn’t change. If it will need to change (like in a for loop), you should use let.

for (let i = 0; i < 5; i++) { console.log(`i is ${i}`), 125/*ms*/); } Specifically, this outputs:Consequently, let variables are less likely to cause problems when used in large programs or when independently-developed frameworks are combined in new and unexpected ways.

// Logs 3 thrice, not what we meant. for (var i = 0; i < 3; i++) { setTimeout(() => console.log(i), 0); } // Logs 0, 1 and 2, as expected. for (let j = 0; j < 3; j++) { setTimeout(() => console.log(j), 0); } Temporal dead zone Because of the temporal dead zone, variables declared using let can't be accessed before they are declared. Attempting to do so throws an error. let msg = "Hello World"; function doWork() { // msg will be available since it was defined above this opening bracket! let friends = 0; console.log(msg); // with VAR though: for (var iCount2 = 0; iCount2 < 5; iCount2++) {} // iCount2 will be available after this closing bracket! console.log(iCount2); for (let iCount1 = 0; iCount1 < 5; iCount1++) {} // iCount1 will not be available behind this closing bracket, it will return undefined console.log(iCount1); } // friends will no be available after this closing bracket! doWork(); console.log(friends); The restrictions imposed by let reduce the visibility of the variables and increase the likelihood that unexpected name collisions will be found early. This makes it easier to track and reason about variables, including their reachability(helping with reclaiming unused memory).console.log(varVar); var varVar = 10; // logs undefined, the variable gets hoisted Global let doesn't get attached to window: A variable declared with let in the global scope (which is code that is not in a function) doesn't get added as a property on the global window object. For example (this code is in global scope):

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What's the difference between let and var?

Today, however, that is definitely not the case. In fact, we need actually to ask ourselves whether it's safe to use the let statement. The answer to that question depends on your environment:let gfoo = 123; if (true) { let gfoo = 456; } console.log(gfoo); // 123 var hfoo = 123; if (true) { var hfoo = 456; } console.log(hfoo); // 456 The gfoo, defined by let initially is in the global scope, and when we declare gfoo again inside the if clause its scope changed and when a new value is assigned to the variable inside that scope it does not affect the global scope.function getDate () { var date = new Date() function formatDate () { return date.toDateString().slice(4) // ✅ } return formatDate() } getDate() console.log(date) // ❌ Reference Error Now let’s look at a more advanced example. Say we had an array of prices and we needed a function that took in that array as well as a discount and returned us a new array of discounted prices. The end goal might look something like this. 3 In most basic terms, for (let i = 0; i < 5; i++) { // i accessible ✔️ } // i not accessible ❌ for (var i = 0; i < 5; i++) { // i accessible ✔️ } // i accessible ✔️ ⚡️ Sandbox to play around ↓ A constant is an identifier for a simple value. The value cannot be modified during the script's execution. In JavaScript, const statement creates a constant. Constants follow the same scope rules as JavaScript variables. Version. The current implementation of const is a Mozilla-specific extension and is not part of ECMAScript 5. Synta

Is it safe to use let today?

Sign in to enjoy the benefits of an MDN account. If you haven’t already created an account, you will be prompted to do so after signing in. function varTest() { var x = 1; { var x = 2; // same variable! console.log(x); // 2 } console.log(x); // 2 } function letTest() { let x = 1; { let x = 2; // different variable console.log(x); // 2 } console.log(x); // 1 } At the top level of programs and functions, let, unlike var, does not create a property on the global object. For example:for (var i = 0; i < 10 ; i++) { setTimeout( function a() { console.log(i); //print 10 times 10 }, 100 * i); } This is because when using var, for every loop iteration the variable is scoped and has shared copy.

Difference between var, let and const in Javascript

function discountPrices (prices, discount) { console.log(discounted) // undefined var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } Now that you know everything there is to know about var, let’s finally talk about the whole point of why you’re here, what’s the difference between var, let, and const? var a; var a; // Works fine. let b; let b; // SyntaxError: Identifier 'b' has already been declared var c; let c; // SyntaxError: Identifier 'c' has already been declared const const is quite similar to let—it's block-scoped and has TDZ. There are, however, two things which are different.Functions often compute a return value. The return value is "returned" back to the "caller":

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const toKebabCase = str => str && str .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g) .map(x => x.toLowerCase()) .join('-'); console.log(toKebabCase('camelCase')); // 'camel-case' console.log(toKebabCase('some text')); // 'some-text' console.log(toKebabCase('some-mixed_string With spaces_underscores-and-hyphens')); // 'some-mixed-string-with-spaces-underscores-and-hyphens' console.log(toKebabCase('AllThe-small Things')); // "all-the-small-things" console.log(toKebabCase('IAmListeningToFMWhileLoadingDifferentURLOnMyBrowserAndAlsoEditingSomeXMLAndHTML')); // "i-am-listening-to-fm-while-loading-different-url-on-my-browser-and-also-editing-xml-and-html" Output:So to recap, var is function scoped and if you try to use a variable declared with var before the actual declaration, you’ll just get undefined. const and let are blocked scoped and if you try to use variable declared with let or const before the declaration you’ll get a ReferenceError. Finally the difference between let and const is that once you’ve assigned a value to const, you can’t reassign it, but with let, you can. "use strict"; console.log("var:"); for (var j = 0; j < 2; j++) { console.log(j); } console.log(j); console.log("let:"); for (let i = 0; i < 2; i++) { console.log(i); } console.log(i); The difference is scoping. var is scoped to the nearest function block and let is scoped to the nearest enclosing block, which can be smaller than a function block. Both are global if outside any block.Lets see an example:

Difference between let and var in Javascript - codeburs

JavaScript is the programming language of HTML and the Web. JavaScript is easy to learn. This tutorial will teach you JavaScript from basic to advanced. With our Try it Yourself editor, you can edit the source code and view the result. We recommend reading this tutorial, in the sequence listed in the left menu ES2015 (or ES6) introduced two new ways to create variables, let and const. But before we actually dive into the differences between var, let, and const, there are some prerequisites you need to know first. They are variable declarations vs initialization, scope (specifically function scope), and hoisting. Now the unpopular opinion, though it still has some validity to it, is that you should never use const because even though you’re trying to signal that the variable is immutable, as we saw above, that’s not entirely the case. Developers who subscribe to this opinion always use let unless they have variables that are actually constants like _LOCATION_ = ....

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ES6からはletが使えるため、今後はvarではなく好んでletが使われるようになるかと思います。 参考 パーフェクトJavaScript |Part2 JavaScript言語仕様 - 6章 関数とクロージャ - 6-4 スコープ - 6-4-3 letとブロックスコー var a = 1; var b = 2; if (a === 1) { var a = 11; // the scope is global let b = 22; // the scope is inside the if-block console.log(a); // 11 console.log(b); // 22 } console.log(a); // 11 console.log(b); // 2 However, this combination of var and let declaration below is a SyntaxError due to var being hoisted to the top of the block. This results in an implicit re-declaration of the variable.

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function varTest() { var x = 1; if (true) { var x = 2; // same variable! console.log(x); // 2 } console.log(x); // 2 } function letTest() { let x = 1; if (true) { let x = 2; // different variable console.log(x); // 2 } console.log(x); // 1 }` At the top level of programs and functions, let, unlike var, does not create a property on the global object. For example:var and variables in ES5 has scopes in functions meaning the variables are valid within the function and not outside the function itself.

146 Here's an explanation of the let keyword with some examples.{ console.log(cc); // undefined. Caused by hoisting var cc = 23; } { console.log(bb); // ReferenceError: bb is not defined let bb = 23; } Actually, Per @Bergi, Both var and let are hoisted. There's a special syntax to work with promises in a more comfortable fashion, called async/await. It's surprisingly easy to understand and use. Async functions. Let's start with the async keyword. It can be placed before a function, like this: async function f () { return 1; } The word async before a function means one simple. Now that we’ve discussed variable declarations, initializations, and scope, the last thing we need to flush out before we dive into let and const is hoisting.

let x = 1; let x = 2; share | improve this answer | follow | | | | edited Nov 28 '16 at 9:31 John Slegers 34.5k1515 gold badges168168 silver badges143143 bronze badges answered Aug 11 '15 at 0:35 RDocRDoc 11722 silver badges77 bronze badges add a comment  |  9 var is global scope (hoist-able) variable.Another difference between var and let is variables with declared with let don't get hoisted. An example is the best way to illustrate this behavior: let promise = fetch (url, [options]) url - the URL to access. options - optional parameters: method, headers etc. Without options, that is a simple GET request, downloading the contents of the url. The browser starts the request right away and returns a promise that the calling code should use to get the result var foo = "Foo"; // globally scoped let bar = "Bar"; // globally scoped console.log(window.foo); // Foo console.log(window.bar); // undefined Redeclaration In strict mode, var will let you re-declare the same variable in the same scope while let raises a SyntaxError.Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

See my example below, and note how the lion (let) variable acts differently in the two console.logs; it becomes out of scope in the 2nd console.log.function discountPrices (prices, discount) { let discounted = [] for (let i = 0; i < prices.length; i++) { let discountedPrice = prices[i] * (1 - discount) let finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } discountPrices([100, 200, 300], .5) // ❌ ReferenceError: i is not defined 🙅‍♀️ We get ReferenceError: i is not defined. What this tells us is that variables declared with let are block scoped, not function scoped. So trying to access i (or discountedPrice or finalPrice) outside of the “block” they were declared in is going to give us a reference error as we just barely saw.function a(){ b; let b; } a(); > Uncaught ReferenceError: b is not defined With a variable defined with var, if the Engine cannot find the variable in the current Execution Context's Variable Environment, then it will go up the scope chain (the Outer Environment) and check the Outer Environment's Variable Environment for the variable. If it cannot find it there, it will continue searching the Scope Chain. This is not the case with let and const. 111 The accepted answer is missing a point:

let, is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it’s defined in, which is not always the entire containing function.“If the variable statement occurs inside a FunctionDeclaration, the variables are defined with function-local scope in that function.”. const person = { name: 'Kim Kardashian' } person.name = 'Kim Kardashian West' // ✅ person = {} // ❌ Assignment to constant variable. Notice that changing a property on an object isn’t reassigning it, so even though an object is declared with const, that doesn’t mean you can’t mutate any of its properties. It only means you can’t reassign it to a new value. In Javascript one can define variables using the keywords var, let or const. var a=10; let b=20; const PI=3.14; var: The scope of a variable defined with the keyword var is limited to the function within which it is defined. If it is defined outside any function, the scope of the variable is global. var is function scoped

Use let over var whenever you can because it is simply scoped more specific. This reduces potential naming conflicts which can occur when dealing with a large number of variables. var can be used when you want a global variable explicitly to be on the window object (always consider carefully if this is really necessary). 1 Now I think there is better scoping of variables to a block of statements using let:

JavaScript promises started out in the DOM as Futures, renamed to Promises, and finally moved into JavaScript. Having them in JavaScript rather than the DOM is great because they'll be available in non-browser JS contexts such as Node.js (whether they make use of them in their core APIs is another question) An arrow function expression is a syntactically compact alternative to a regular function expression, although without its own bindings to the this, arguments, super, or new.target keywords. Arrow function expressions are ill suited as methods, and they cannot be used as constructors

The second feature of let is it introduces block scope. Blocks are defined by curly braces. Examples include function blocks, if blocks, for blocks, etc. When you declare a variable with let inside of a block, the variable is only available inside of the block. In fact, each time the block is run, such as within a for loop, it will create a new variable in memory.In contract to variable declaration, variable initialization is when you first assign a value to a variable. function discountPrices (prices, discount) { console.log(discounted) // ❌ ReferenceError let discounted = [] for (let i = 0; i < prices.length; i++) { let discountedPrice = prices[i] * (1 - discount) let finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } var VS let var: function scoped undefined when accessing a variable before it's declared let: block scoped ReferenceError when accessing a variable before it's declared let VS const Now that you understand the difference between var and let, what about const? Turns out, const is almost exactly the same as let. However, the only difference is that once you’ve assigned a value to a variable using const, you can’t reassign it to a new value.

Exercise 1 »  Exercise 2 »  Exercise 3 »  Exercise 4 »  Exercise 5 »  let x = 1; switch(x) { case 0: { let foo; break; } case 1: { let foo; break; } } Temporal dead zone Unlike variables declared with var, which will start with the value undefined, let variables are not initialized until their definition is evaluated. Accessing the variable before the initialization results in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until the initialization is processed.var variables are global and can be accessed basically everywhere, while let variables are not global and only exist until a closing parenthesis kills them. var funcs = []; // let's create 3 functions for (var i = 0; i < 3; i++) { // and store them in funcs funcs[i] = function() { // each should log its value. console.log("My value: " + i); }; } for (var j = 0; j < 3; j++) { // and now let's run each one to see funcs[j](); } My value: 3 was output to console each time funcs[j](); was invoked since anonymous functions were bound to the same variable.

function do_something() { console.log(bar); // undefined console.log(foo); // ReferenceError var bar = 1; let foo = 2; } The temporal dead zone and typeof Unlike with simply undeclared variables and variables that hold a value of undefined, using the typeof operator to check for the type of a variable in that variable's temporal dead zone will throw a ReferenceError:(() => { var functionScopedVariable = 42; let blockScopedVariable = 43; console.log(functionScopedVariable); // 42 console.log(blockScopedVariable); // 43 })(); console.log(functionScopedVariable); // ReferenceError: functionScopedVariable is not defined console.log(blockScopedVariable); // ReferenceError: blockScopedVariable is not defined Inside a block Variables declared using let inside a block can't be accessed outside that block.var bar = 5; let foo = 10; console.log(bar); // logs 5 console.log(foo); // logs 10 console.log(window.bar); // logs 5, variable added to window object console.log(window.foo); // logs undefined, variable not added to window object In my both examples I have a function myfunc. myfunc contains a variable myvar equals to 10. In my first example I check if myvar equals to 10 (myvar==10) . If yes, I agian declare a variable myvar (now I have two myvar variables)using var keyword and assign it a new value (20). In next line I print its value on my console. After the conditional block I again print the value of myvar on my console. If you look at the output of myfunc, myvar has value equals to 20. function test(){ var foo = 33; if(foo) { let foo = (foo + 55); // ReferenceError } } test(); This phenomenon may confuse you in a situation like the following. The instruction let n of n.a is already inside the private scope of the for loop's block. So, the identifier n.a is resolved to the property 'a' of the 'n' object located in the first part of the instruction itself (let n).

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var Thing; { let privateScope = new WeakMap(); let counter = 0; Thing = function() { this.someProperty = 'foo'; privateScope.set(this, { hidden: ++counter, }); }; Thing.prototype.showPublic = function() { return this.someProperty; }; Thing.prototype.showPrivate = function() { return privateScope.get(this).hidden; }; } console.log(typeof privateScope); // "undefined" var thing = new Thing(); console.log(thing); // Thing {someProperty: "foo"} thing.showPublic(); // "foo" thing.showPrivate(); // 1 The same privacy pattern with closures over local variables can be created with var, but those need a function scope (typically an IIFE in the module pattern) instead of just a block scope like in the example above.People had to create immediately invoked functions to capture correct value from the loops but that was also hairy.let name = 'Tyler' const handle = 'tylermcginnis' name = 'Tyler McGinnis' // ✅ handle = '@tylermcginnis' // ❌ TypeError: Assignment to constant variable. The take away above is that variables declared with let can be re-assigned, but variables declared with const can’t be. First, let’s compare var and let. The main difference between var and let is that instead of being function scoped, let is block scoped. What that means is that a variable created with the let keyword is available inside the “block” that it was created in as well as any nested blocks. When I say “block”, I mean anything surrounded by a curly brace {} like in a for loop or an if statement. for (var i = 0; i < 3; i++) { var j = i * 2; } console.log(i); // 3 console.log(j); // 4 for (let k = 0; k < 3; k++) { let l = k * 2; } console.log(typeof k); // undefined console.log(typeof l); // undefined // Trying to do console.log(k) or console.log(l) here would throw a ReferenceError. Loops with closures If you use let instead of var in a loop, with each iteration you get a new variable. That means that you can safely use a closure inside a loop.

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