JavaScript Let keyword
这 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:
例子
let x = "John Doe";
let x = 0;
// SyntaxError: 'x' has already been declared
With var you can:
例子
var x = "John Doe";
var x = 0;
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:
例子
{
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.
例子
{
var x = 2;
}
// x CAN be used here
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:
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:
Browser Support
这 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 |
Redeclaring
Redeclaring a JavaScript variable with var is allowed anywhere in a program:
With let, redeclaring a variable in the same block is NOT allowed:
例子
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:
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:
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:
