JavaScript let 변수

https://www.w3schools.com/js/js_let.asp

JavaScript Let

The let keyword was introduced in ES6 (2015).

Variables defined with let cannot be Redeclared.

Variables defined with let must be Declared before use.

Variables defined with let have Block Scope.

Cannot be Redeclared

Variables defined with let cannot be redeclared.

You cannot accidentally redeclare a variable.

With let you can not do this:

Example

let x = "John Doe";

let x = 0;

// SyntaxError: 'x' has already been declared

With var you can:

Example

var x = "John Doe";

var x = 0;

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Block Scope

Before ES6 (2015), JavaScript had only Global Scope and Function Scope.

ES6 introduced two important new JavaScript keywords: let and const.

These two keywords provide Block Scope in JavaScript.

Variables declared inside a { } block cannot be accessed from outside the block:

Example

{
  let x = 2;
}
// x can NOT be used here

Variables declared with the var keyword can NOT have block scope.

Variables declared inside a { } block can be accessed from outside the block.

Example

{
  var x = 2;
}
// x CAN be used here

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Redeclaring Variables

Redeclaring a variable using the var keyword can impose problems.

Redeclaring a variable inside a block will also redeclare the variable outside the block:

Example

var x = 10;
// Here x is 10

{
var x = 2;
// Here x is 2
}

// Here x is 2

Try it Yourself »

Redeclaring a variable using the let keyword can solve this problem.

Redeclaring a variable inside a block will not redeclare the variable outside the block:

Example

let x = 10;
// Here x is 10

{
let x = 2;
// Here x is 2
}

// Here x is 10

Try it Yourself »

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Browser Support

The let keyword is not fully supported in Internet Explorer 11 or earlier.

The following table defines the first browser versions with full support for the let keyword:

Chrome 49	Edge 12	Firefox 44	Safari 11	Opera 36
Mar, 2016	Jul, 2015	Jan, 2015	Sep, 2017	Mar, 2016

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Redeclaring

Redeclaring a JavaScript variable with var is allowed anywhere in a program:

Example

var x = 2;
// Now x is 2

var x = 3;
// Now x is 3

Try it Yourself »

With let, redeclaring a variable in the same block is NOT allowed:

Example

var x = 2;    // Allowed
let x = 3;    // Not allowed

{
let x = 2;    // Allowed
let x = 3     // Not allowed
}

{
let x = 2;    // Allowed
var x = 3     // Not allowed
}

Redeclaring a variable with let, in another block, IS allowed:

Example

let x = 2;    // Allowed

{
let x = 3;    // Allowed
}

{
let x = 4;    // Allowed
}

Try it Yourself »

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Let Hoisting

Variables defined with var are hoisted to the top and can be initialized at any time.

Meaning: You can use the variable before it is declared:

Example

This is OK:

carName = "Volvo";
var carName;

Try it Yourself »

If you want to learn more about hoisting, study the chapter JavaScript Hoisting.

Variables defined with let are also hoisted to the top of the block, but not initialized.

Meaning: Using a let variable before it is declared will result in a ReferenceError:

Example

carName = "Saab";
let carName = "Volvo";