What's the difference between using “let” and

Question

ECMAScript 6 introduced the let statement. I've heard it described as a "local" variable, but I'm still not quite sure how it behaves differently than the var keyword.

What are the differences? When should let be used over var?

Answer 1

let is a part of es6. These functions will explain the difference in easy way.

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
}

Answer 2

The main difference is the scope difference, while let can be only available inside the scope it's declared, like in for loop, var can be accessed outside the loop for example. From the documentation in MDN (examples also from MDN):

let allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used. This is unlike the var keyword, which defines a variable globally, or locally to an entire function regardless of block scope.

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:

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 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.

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

Answer 3

This article clearly defines the difference between var, let and const

const is a signal that the identifier won’t be reassigned.

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.

var is now the weakest signal available when you define a variable in JavaScript. The variable may or may not be reassigned, and the variable may or may not be used for an entire function, or just for the purpose of a block or loop.

https://medium.com/javascript-scene/javascript-es6-var-let-or-const-ba58b8dcde75#.esmkpbg9b

Answer 4

let

Block scope

Variables declared using the let keyword are block-scoped, which means that they are available only in the block in which they were declared.

At the top level (outside of a function)

At the top level, variables declared using let don't create properties on the global object.

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.

(() => {
  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 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.

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.

// 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.

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.

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.

No re-assigning

Variable declared using const can't be re-assigned.

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.

const obj = {};
obj.a = 42;
console.log(obj.a); // 42

If you want to have an immutable object, you should use Object.freeze().

Initializer is required

You always must specify a value when declaring a variable using const.

const a; // SyntaxError: Missing initializer in const declaration

Answer 5

There are some subtle differences -- let scoping behaves more like variable scoping does in more or less any other languages.

e.g. It scopes to the enclosing block, They don't exist before they're declared, etc.

However it's worth noting that let is only a part of newer Javascript implementations and has varying degrees of browser support.

Answer 6

Here's an example to add on to what others have already written. Suppose you want to make an array of functions, adderFunctions, where each function takes a single Number argument and returns the sum of the argument and the function's index in the array. Trying to generate adderFunctions with a loop using the var keyword won't work the way someone might naïvely expect:

// 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.

However, we can try again using the let keyword:

// 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.

Now, image mixing the two behaviors and you'll probably see why it's not recommended to mix the newer let and const with the older var in the same script. Doing so can result is some spectacularly confusing code.

const doubleAdderFunctions = [];

for (var i = 0; i < 1000; i++) {
    const j = i;
    doubleAdderFunctions[i] = x => x + i + j;
}

const add18 = doubleAdderFunctions[9];
const add24 = doubleAdderFunctions[12];

// It's not fun debugging situations like this, especially when the
// code is more complex than in this example.
console.log(add18(24) === 42); // => false
console.log(add24(18) === 42); // => false
console.log(add18(24) === add24(18)); // => false
console.log(add18(24) === 2018); // => false
console.log(add24(18) === 2018); // => false
console.log(add18(24) === 1033); // => true
console.log(add24(18) === 1030); // => true

Don't let this happen to you. Use a linter.

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.