Javescript——变量声明的区别

原文链接：ES6 Syntax and Feature Overview

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Keyword	Scope	Hoisting	Can Be Reassigned	Can Be Redeclared
var	Function scope	Yes	Yes	Yes
let	Block scope	No	Yes	No
const	Block scope	No	No	No

 Understanding Variables, Scope, and Hoisting in JavaScript

 In JavaScript, there are three keywords used to declare a variable — var, let, and const — and each one affects how the code will interpret the variable differently. 

 In JavaScript, the value contained inside a variable can be any JavaScript data type, including a number, string, or object. 

We can use var to demonstrate the concept of a variable itself. 

As mentioned previously, variables can be used to represent any JavaScript data type. In this example, we’ll declare variables with string, number, object, Boolean, and null values.

Variables store data in memory which can later be accessed and modified. Variables can also be reassigned and given a new value.

Naming Variables

Understanding Syntax and Code Structure in JavaScript,

JavaScript also has the convention of using camel case (sometimes stylized as camelCase) in the names of functions and variables declared with var or let. This is the practice of writing the first word lowercase, and then capitalizing the first letter of every subsequent word with no spaces between them.

Most variables that are not constants will follow this convention, with some exceptions. The names of variables that are constant, declared with the const keyword, are typically written in all uppercase. 

Difference Between var, let, and const

JavaScript has three different keywords to declare a variable, which adds an extra layer of intricacy to the language. The differences between the three are based on scope, hoisting, and reassignment.

Keyword	Scope	Hoisting	Can Be Reassigned	Can Be Redeclared
var	Function scope	Yes	Yes	Yes
let	Block scope	No	Yes	No
const	Block scope	No	No	No

You may be wondering which of the three you should use in your own programs. A commonly accepted practice is to use const as much as possible, and let in the case of loops and reassignment. Generally, var can be avoided outside of working on legacy code.

Variable Scope

Scope in JavaScript refers to the current context of code, which determines the accessibility of variables to JavaScript. The two types of scope are local and global:

• Global variables are those declared outside of a block
• Local variables are those declared inside of a block

We learned that variables can be reassigned. Using local scope, we can actually create new variables with the same name as a variable in an outer scope without changing or reassigning the original value.

In the example below, we will create a global species variable. Within the function is a local variable with the same name. By sending them to the console, we can see how the variable’s value is different depending on the scope, and the original value is not changed.

// Initialize a global variable
var species = "human";

function transform() {
  // Initialize a local, function-scoped variable
  var species = "werewolf";
  console.log(species);
}

// Log the global and local variable
console.log(species);
transform();
console.log(species);

　

#Output
human
werewolf
human

In this example, the local variable is function-scoped. Variables declared with the var keyword are always function-scoped, meaning they recognize functions as having a separate scope. This locally-scoped variable is therefore not accessible from the global scope.

The new keywords let and const, however, are block-scoped. This means that a new, local scope is created from any kind of block, including function blocks, if statements, and for and while loops.

To illustrate the difference between function- and block-scoped variables, we will assign a new variable in an if block using let.

var fullMoon = true;

// Initialize a global variable
let species = "human";

if (fullMoon) {
  // Initialize a block-scoped variable
  let species = "werewolf";
  console.log(`It is a full moon. Lupin is currently a ${species}.`);
}

console.log(`It is not a full moon. Lupin is currently a ${species}.`);

　　

#Output
It is a full moon. Lupin is currently a werewolf.
It is not a full moon. Lupin is currently a human.

　　

In this example, the species variable has one value globally (human), and another value locally (werewolf). If we were to use var, however, there would be a different result.

// Use var to initialize a variable
var species = "human";

if (fullMoon) {
  // Attempt to create a new variable in a block
  var species = "werewolf";
  console.log(`It is a full moon. Lupin is currently a ${species}.`);
}

console.log(`It is not a full moon. Lupin is currently a ${species}.`);

　　

# Output
It is a full moon. Lupin is currently a werewolf.
It is not a full moon. Lupin is currently a werewolf.

　In the result of this example, both the global variable and the block-scoped variable end up with the same value, werewolf. This is because instead of creating a new local variable with var, you are reassigning the same variable in the same scope. var does not recognize if to be part of a different, new scope

　It is generally recommended that you declare variables that are block-scoped, as they produce code that is less likely to unintentionally override variable values.

Hoisting

Javascript Hoisting（国内一般翻译为 变量提升）

Javascript Hoisting：In javascript, every variable declaration is hoisted to the top of its declaration context.我的理解就是在Javascript语言中，变量的声明(注意不包含变量初始化）会被提升（置顶）到声明所在的上下文，也就是说，在变量的作用域内，不管变量在何处声明，都会被提升到作用域的顶部，但是变量初始化的顺序不变。

If we attempt to use a variable before it has been declared and initialized, it will return undefined.

// Attempt to use a variable before declaring it
console.log(x);

// Variable assignment
var x = 100;

　　

# Output
undefined

However, if we omit the var keyword, we are no longer declaring the variable, only initializing it. It will return a ReferenceError and halt the execution of the script.　　

// Attempt to use a variable before declaring it
console.log(x);

// Variable assignment without var
x = 100;

　　

# Output
ReferenceError: x is not defined

　　

The reason for this is due to hoisting, a behavior of JavaScript in which variable and function declarations are moved to the top of their scope.

 Since only the actual declaration is hoisted, not the initialization, the value in the first example returns undefined.

Keyword	Scope	Hoisting	Can Be Reassigned	Can Be Redeclared
var	Function scope	Yes	Yes	Yes
let	Block scope	No	Yes	No
const	Block scope	No	No	No

　　

To demonstrate this concept more clearly, below is the code we wrote and how JavaScript actually interpreted it.

// The code we wrote
console.log(x);
var x = 100;

// How JavaScript interpreted it
var x;
console.log(x);
x = 100;

　　JavaScript saved x to memory as a variable before the execution of the script. Since it was still called before it was defined, the result is undefined and not 100. However, it does not cause a ReferenceError and halt the script. Although the var keyword did not actually change location of the var, this is a helpful representation of how hoisting works.

This behavior can cause issues, though, because the programmer who wrote this code likely expects the output of x to be true, when it is instead undefined. 

We can also see how hoisting can lead to unpredictable results in the next example:

// Initialize x in the global scope
var x = 100;

function hoist() {
  // A condition that should not affect the outcome of the code
  if (false) {
    var x = 200;
  }
  console.log(x);
}

hoist();

　　

# Output
undefined

　　In this example, we declared x to be 100 globally. Depending on an if statement, x could change to 200, but since the condition was false it should not have affected the value of x. Instead, x was hoisted to the top of the hoist() function, and the value became undefined. 

This type of unpredictable behavior can potentially cause bugs in a program. Since let and const are block-scoped, they will not hoist in this manner, as seen below.

// Initialize x in the global scope
let x = true;

function hoist() {
  // Initialize x in the function scope
  if (3 === 4) {
    let x = false;
  }
  console.log(x);
}

hoist();

　　

# Output
true

　　

Duplicate declaration of variables, which is possible with var, will throw an error with let and const.

// Attempt to overwrite a variable declared with var
var x = 1;
var x = 2;

console.log(x);

# Output
2

// Attempt to overwrite a variable declared with let
let y = 1;
let y = 2;

console.log(y);

# Output
Uncaught SyntaxError: Identifier 'y' has already been declared

　　To summarize, variables introduced with var have the potential of being affected by hoisting, a mechanism in JavaScript in which variable declarations are saved to memory. This may result in undefined variables in one’s code.

The introduction of let and const resolves this issue by throwing an error when attempting to use a variable before declaring it or attempting to declare a variable more than once.

Constants

Many programming languages feature constants, which are values that cannot be modified or changed. In JavaScript, the const identifier is modelled after constants, and the values assigned to a const cannot be reassigned.

It is common convention to write all const identifiers in uppercase. This marks them as readily distinguishable from other variable values.

In the example below, we initialize the variable SPECIES as a constant with the const keyword. Trying to reassign the variable will result in an error. 

// Assign value to const
const SPECIES = "human"; 

// Attempt to reassign value
SPECIES = "werewolf";

console.log(SPECIES);

# Output
Uncaught TypeError: Assignment to constant variable.

　　Since const values cannot be reassigned, they need to be declared and initialized at the same time, or will also throw an error.

// Declare but do not initialize a const
const TODO;

console.log(TODO);

Output
Uncaught SyntaxError: Missing initializer in const declaration

　　

Values that cannot change in programming are known as immutable, while values that can be changed are mutable.

Although const values cannot be reassigned, they are mutable as it is possible to modify the properties of objects declared with const.

// Create a CAR object with two properties
const CAR = {
    color: "blue",
    price: 15000
}

// Modify a property of CAR
CAR.price = 20000;

console.log(CAR);

# Output
{ color: 'blue', price: 20000 }

　Constants are useful for making it clear to your future self and other programmers working on a project with you that the intended variable should not be reassigned. If you expect that a variable may be modified in the future, you will likely want to use let to declare the variable instead. 

　To compare how variables are used in other languages, you can read our tutorial on “How To Use Variables in Python 3.”

　

MDN Reference: let

 The let statement declares a block scope local variable, optionally initializing it to a value.

let x = 8;

if (x === 8) {
  let x = 4;

  console.log(x);
  // expected output: 2
}

console.log(x);
// expected output: 1

　　

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

The other difference between var and let is that the latter is initialized to value only when parser evaluates it (see below).

 Just like const the let does not create properties of the window object when declared globally (in the top-most scope).

An explanation of why the name "let" was chosen can be found here.

• var takes that name - it is variable,
• const - it is a constant,
• Let is a mathematical statement that was adopted by early programming languages like Scheme and Basic.

Variables are considered low level entities not suitable for higher levels of abstraction, thus the desire of many language designers to introduce similar but more powerful concepts like in Clojure, F#, Scala, where let might mean a value, or a variable that can be assigned, but not changed, which in turn lets the compiler catch more programming errors and optimize code better.

JavaScript has had var from the beginning, so they just needed another keyword, and just borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible, although in JavaScript let creates block scope local variable instead. 

使用 var 关键字声明的变量不具备块级作用域的特性，它在 {} 外依然能被访问到。

在 ES6 之前，是没有块级作用域的概念的。

ES6 可以使用 let 关键字来实现块级作用域。

let 声明的变量只在 let 命令所在的代码块 {} 内有效，在 {} 之外不能访问。

Scoping rules

Variables declared by let have their scope in the block for 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;
  {
    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:

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:

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.

Redeclarations

Redeclaring the same variable within the same function or block scope raises a SyntaxError.

Keyword	Scope	Hoisting	Can Be Reassigned	Can Be Redeclared
var	Function scope	Yes	Yes	Yes
let	Block scope	No	Yes	No
const	Block scope	No	No	No

if (x) {
  let foo;
  let foo; // SyntaxError thrown.
}

　　

You may encounter errors in switch statements because there is only one block.

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.

let x = 1;

switch(x) {
  case 0: {
    let foo;
    break;
  }  
  case 1: {
    let foo;
    break;
  }
}

　　

Temporal dead zone

hoisting

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.

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:

// 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.

In the same line, the if block's foo has already been created in the lexical environment, but has not yet reached (and terminated) its initialization (which is part of the statement itself). 

The if block's foo is still in the temporal dead zone.

function test(){
   var foo = 33;
   {
      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 identifiern.a is resolved to the property 'a' of the 'n' object located in the first part of the instruction itself ("let n"). 

This is still in the temporal dead zone as its declaration statement has not been reached and terminated.

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 varwhose 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

　　

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.(hoisting)

let x = 1;

{
  var x = 2; // SyntaxError for re-declaration
}

　　

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const

Constants are block-scoped, much like variables defined using the let statement. The value of a constant can't be changed through reassignment, and it can't be redeclared.

const number = 42;

try {
  number = 99;
} catch(err) {
  console.log(err);
  // expected output: TypeError: invalid assignment to const `number'
  // Note - error messages will vary depending on browser
}

console.log(number);
// expected output: 42

　　

Syntax

const name1 = value1 [, name2 = value2 [, ... [, nameN = valueN]]];

nameN

The constant's name, which can be any legal identifier.

valueN

The constant's value; this can be any legal expression, including a function expression.

 

Description

This declaration creates a constant whose scope can be either global or local to the block in which it is declared.

Global constants do not become properties of the window object, unlike var variables.

An initializer for a constant is required; that is, you must specify its value in the same statement in which it's declared (which makes sense, given that it can't be changed later).

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in the case where the content is an object, this means the object's contents (e.g., its properties) can be altered.

All the considerations about the "temporal dead zone" apply to both let and const.(hoisting)

A constant cannot share its name with a function or a variable in the same scope.

Examples

The following example demonstrates how constants behave. Try this in your browser console.

// NOTE: Constants can be declared with uppercase or lowercase, but a common
// convention is to use all-uppercase letters.

// define MY_FAV as a constant and give it the value 7
const MY_FAV = 7;

// this will throw an error - Uncaught TypeError: Assignment to constant variable.
MY_FAV = 20;

// MY_FAV is 7
console.log('my favorite number is: ' + MY_FAV);

// trying to redeclare a constant throws an error -  Uncaught SyntaxError: Identifier 'MY_FAV' has already been declared
const MY_FAV = 20;

// the name MY_FAV is reserved for constant above, so this will fail too
var MY_FAV = 20;

// this throws an error too
let MY_FAV = 20;

// it's important to note the nature of block scoping
if (MY_FAV === 7) { 
    // this is fine and creates a block scoped MY_FAV variable 
    // (works equally well with let to declare a block scoped non const variable)
    let MY_FAV = 20;

    // MY_FAV is now 20
    console.log('my favorite number is ' + MY_FAV);

    // this gets hoisted into the global context and throws an error
    var MY_FAV = 20;
}

// MY_FAV is still 7
console.log('my favorite number is ' + MY_FAV);

// throws an error - Uncaught SyntaxError: Missing initializer in const declaration
const FOO; 

// const also works on objects
const MY_OBJECT = {'key': 'value'};

// Attempting to overwrite the object throws an error - Uncaught TypeError: Assignment to constant variable.
MY_OBJECT = {'OTHER_KEY': 'value'};

// However, object keys are not protected,
// so the following statement is executed without problem
MY_OBJECT.key = 'otherValue'; // Use Object.freeze() to make object immutable

// The same applies to arrays
const MY_ARRAY = [];
// It's possible to push items into the array
MY_ARRAY.push('A'); // ["A"]
// However, assigning a new array to the variable throws an error - Uncaught TypeError: Assignment to constant variable.
MY_ARRAY = ['B'];

　　

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