The differences between var, let, and const

• Variables declared by var are mounted to window, while variables declared by let and const are not

var a = 'foo'
console.log(window.a) // foo

let b = 'bar'
console.log(window.b) // undefined

const c = 'baz'
console.log(window.c) // undefined
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• Var has variable promotion, but let and const do not. The area above let and const is called a “temporary dead zone” and cannot be used before definition

console.log(a) // undefined
var a = 'foo'

console.log(b) // test4.html:14 Uncaught ReferenceError: Cannot access 'b' before initialization
let b = 'bar'

console.log(c) // test4.html:14 Uncaught ReferenceError: Cannot access 'c' before initialization
const c = 'baz'
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• Declarations for let and const form block-level scope

if (true) {
    var a = 'foo'
    let b = 'bar'
}
console.log(a) // foo
console.log(b) // Uncaught ReferenceError: b is not defined

if (true) {
    var a = 'foo'
    const c = 'baz'
}
console.log(a) // foo
console.log(c) // Uncaught ReferenceError: c is not defined
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• In the same scope, let and const cannot declare the same variable again. Var can

var a = 'foo'
var a = 'bar'
console.log(a) // bar

let b = 'foo'
let b = 'bar'
console.log(b) // test4.html:34 Uncaught SyntaxError: Identifier 'b' has already been declared

const c = 'foo'
const c = 'bar'
console.log(c) // Uncaught SyntaxError: Identifier 'c' has already been declared
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• Let can be modified after it defines a variable, while const appears to declare a “constant.” Const does not guarantee that the value of a variable cannot be changed, but rather that the memory address to which the variable points cannot be changed. For simple data types (Number, String, Boolean), the value is stored in the memory address that the variable points to and is therefore equivalent to a constant; For complex data types (mainly objects and arrays), the variables simply hold the addresses to the heap memory, and it is not possible to control whether the data in the heap is mutable.

const person = {}
person.name = 'maoxiaoxing'
console.log(person) // {name: "maoxiaoxing"}

person = {name: 'king'} // test4.html:45 Uncaught TypeError: Assignment to constant variable.

const a = ['foo']
a.push('bar')
a[2] = 'baz'
console.log(a) // ["foo", "bar", "baz"]
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