const

常量是块范围的，就像使用let语句定义的变量一样。常量的值不能通过重新赋值而改变，并且不能重新声明。

语法

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

nameN常量的名称，可以是任何合法的标识符。valueN常数值; 这可以是任何法律表达式，包括函数表达式。

描述

该声明创建一个常量，其作用域可以是全局的（在窗口对象上），也可以是声明它的块的本地。需要一个常量的初始值; 也就是说，您必须在声明它的同一个语句中指定它的值（这是有道理的，因为它以后不能更改）。

在const创建了一个只读的参照值。它并不意味着其持有的价值是不变的，只是该变量标识符不能被重新分配。例如，在内容是对象的情况下，这意味着对象的内容（例如，其参数）可以被改变。

关于“时间死区”的所有事项适用于let和const。

常量不能在同一范围内与函数或变量共享其名称。

示例

以下示例演示常量的行为。在浏览器控制台中试试这个。

// 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
MY_FAV = 20;

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

// trying to redeclare a constant throws an error
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, 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
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
MY_ARRAY = ['B'];

规范

浏览器兼容性