const

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

句法

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.

描述

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. You must specify its value in the same statement in which it's declared. (This makes sense, given that it can't be changed later.)

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 .

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

范例

Basic const usage

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;
					

Block scoping

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);
					

const needs to be initialized

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

const in objects and arrays

const also works on objects and arrays.

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'];
					

规范

规范
ECMAScript (ECMA-262)
The definition of 'Let and Const Declarations' in that specification.

浏览器兼容性

更新 GitHub 上的兼容性数据
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
const Chrome 21 Edge 12 Firefox 36
36
Prior to Firefox 13, const is implemented, but re-assignment is not failing.
Prior to Firefox 46, a TypeError was thrown on redeclaration instead of a SyntaxError .
IE 11 Opera 9 Safari 5.1 WebView Android ≤37 Chrome Android 25 Firefox Android 36
36
Prior to Firefox 13, const is implemented, but re-assignment is not failing.
Prior to Firefox 46, a TypeError was thrown on redeclaration instead of a SyntaxError .
Opera Android 10.1 Safari iOS 6 Samsung Internet Android 1.5 nodejs 6.0.0

图例

完整支持
完整支持
见实现注意事项。

另请参阅

Metadata

  1. JavaScript
  2. 教程:
  3. 完整初学者
    1. JavaScript 基础
    2. JavaScript 第一步
    3. JavaScript 构建块
    4. 引入 JavaScript 对象
  4. JavaScript 指南
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    2. 语法和类型
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    5. 函数
    6. 表达式和运算符
    7. 数字和日期
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    10. Indexed collections
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    12. Working with objects
    13. 对象模型的细节
    14. Using promises
    15. 迭代器和生成器
    16. Meta programming
    17. JavaScript 模块
  5. 中间体
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    26. Left shift (<<)
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    30. Logical AND (&&)
    31. Logical AND assignment (&&=)
    32. Logical NOT (!)
    33. Logical OR (||)
    34. Logical OR assignment (||=)
    35. Logical nullish assignment (??=)
    36. Multiplication (*)
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    23. SyntaxError: a declaration in the head of a for-of loop can't have an initializer
    24. SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
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    26. SyntaxError: function statement requires a name
    27. SyntaxError: identifier starts immediately after numeric literal
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    30. SyntaxError: missing ) after argument list
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    32. SyntaxError: missing : after property id
    33. SyntaxError: missing ; before statement
    34. SyntaxError: missing = in const declaration
    35. SyntaxError: missing ] after element list
    36. SyntaxError: missing formal parameter
    37. SyntaxError: missing name after . operator
    38. SyntaxError: missing variable name
    39. SyntaxError: missing } after function body
    40. SyntaxError: missing } after property list
    41. SyntaxError: redeclaration of formal parameter "x"
    42. SyntaxError: return not in function
    43. SyntaxError: test for equality (==) mistyped as assignment (=)?
    44. SyntaxError: unterminated string literal
    45. TypeError: "x" has no properties
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    47. TypeError: "x" is not a constructor
    48. TypeError: "x" is not a function
    49. TypeError: "x" is not a non-null object
    50. TypeError: "x" is read-only
    51. TypeError: 'x' is not iterable
    52. TypeError: More arguments needed
    53. TypeError: Reduce of empty array with no initial value
    54. TypeError: X.prototype.y called on incompatible type
    55. TypeError: can't access dead object
    56. TypeError: can't access property "x" of "y"
    57. TypeError: can't assign to property "x" on "y": not an object
    58. TypeError: can't define property "x": "obj" is not extensible
    59. TypeError: can't delete non-configurable array element
    60. TypeError: can't redefine non-configurable property "x"
    61. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    62. TypeError: cyclic object value
    63. TypeError: invalid 'instanceof' operand 'x'
    64. TypeError: invalid Array.prototype.sort argument
    65. TypeError: invalid arguments
    66. TypeError: invalid assignment to const "x"
    67. TypeError: property "x" is non-configurable and can't be deleted
    68. TypeError: setting getter-only property "x"
    69. TypeError: variable "x" redeclares argument
    70. URIError: malformed URI sequence
    71. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    72. Warning: 08/09 is not a legal ECMA-262 octal constant
    73. Warning: Date.prototype.toLocaleFormat is deprecated
    74. Warning: JavaScript 1.6's for-each-in loops are deprecated
    75. Warning: String.x is deprecated; use String.prototype.x instead
    76. Warning: expression closures are deprecated
    77. Warning: unreachable code after return statement
  13. 杂项
    1. JavaScript technologies overview
    2. 词汇语法
    3. JavaScript 数据结构
    4. Enumerability and ownership of properties
    5. Iteration protocols
    6. 严格模式
    7. Transitioning to strict mode
    8. Template literals
    9. 弃用特征