for...of

for...of statement creates a loop iterating over iterable objects , including: built-in String , Array , array-like objects (e.g., arguments or NodeList ), TypedArray , Map , Set , and user-defined iterables. It invokes a custom iteration hook with statements to be executed for the value of each distinct property of the object.

句法

for (variable of iterable) {
  statement
}
					
variable
On each iteration a value of a different property is assigned to variable . variable may be declared with const , let ,或 var .
iterable
Object whose iterable properties are iterated.

范例

Iterating over an Array

const iterable = [10, 20, 30];
for (const value of iterable) {
  console.log(value);
}
// 10
// 20
// 30
					

可以使用 let instead of const too, if you reassign the variable inside the block.

const iterable = [10, 20, 30];
for (let value of iterable) {
  value += 1;
  console.log(value);
}
// 11
// 21
// 31
					

Iterating over a String

const iterable = 'boo';
for (const value of iterable) {
  console.log(value);
}
// "b"
// "o"
// "o"
					

Iterating over a TypedArray

const iterable = new Uint8Array([0x00, 0xff]);
for (const value of iterable) {
  console.log(value);
}
// 0
// 255
					

Iterating over a Map

const iterable = new Map([['a', 1], ['b', 2], ['c', 3]]);
for (const entry of iterable) {
  console.log(entry);
}
// ['a', 1]
// ['b', 2]
// ['c', 3]
for (const [key, value] of iterable) {
  console.log(value);
}
// 1
// 2
// 3
					

Iterating over a Set

const iterable = new Set([1, 1, 2, 2, 3, 3]);
for (const value of iterable) {
  console.log(value);
}
// 1
// 2
// 3
					

Iterating over the arguments object

You can iterate over the arguments object to examine all of the parameters passed into a JavaScript function:

(function() {
  for (const argument of arguments) {
    console.log(argument);
  }
})(1, 2, 3);
// 1
// 2
// 3
					

Iterating over a DOM collection

Iterating over DOM collections like NodeList : the following example adds a read class to paragraphs that are direct descendants of an article:

// Note: This will only work in platforms that have
// implemented NodeList.prototype[Symbol.iterator]
const articleParagraphs = document.querySelectorAll('article > p');
for (const paragraph of articleParagraphs) {
  paragraph.classList.add('read');
}
					

Closing iterators

In for...of loops, abrupt iteration termination can be caused by break , throw or return . In these cases, the iterator is closed.

function* foo(){
  yield 1;
  yield 2;
  yield 3;
};
for (const o of foo()) {
  console.log(o);
  break; // closes iterator, execution continues outside of the loop
}
console.log('done');
					

Iterating over generators

You can also iterate over generators , i.e. functions generating an iterable object:

function* fibonacci() { // a generator function
  let [prev, curr] = [0, 1];
  while (true) {
    [prev, curr] = [curr, prev + curr];
    yield curr;
  }
}
for (const n of fibonacci()) {
  console.log(n);
  // truncate the sequence at 1000
  if (n >= 1000) {
    break;
  }
}
					

Do not reuse generators

Generators should not be re-used, even if the for...of loop is terminated early, for example via the break keyword. Upon exiting a loop, the generator is closed and trying to iterate over it again does not yield any further results.

const gen = (function *(){
  yield 1;
  yield 2;
  yield 3;
})();
for (const o of gen) {
  console.log(o);
  break;  // Closes iterator
}
// The generator should not be re-used, the following does not make sense!
for (const o of gen) {
  console.log(o); // Never called.
}
					

Iterating over other iterable objects

You can also iterate over an object that explicitly implements the iterable 协议:

const iterable = {
  [Symbol.iterator]() {
    return {
      i: 0,
      next() {
        if (this.i < 3) {
          return { value: this.i++, done: false };
        }
        return { value: undefined, done: true };
      }
    };
  }
};
for (const value of iterable) {
  console.log(value);
}
// 0
// 1
// 2
					

Difference between for...of and for...in

Both for...in and for...of statements iterate over something. The main difference between them is in what they iterate over.

for...in statement iterates over the enumerable properties of an object, in an arbitrary order.

for...of statement iterates over values that the iterable object defines to be iterated over.

The following example shows the difference between a for...of loop and a for...in loop when used with an Array .

Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};
const iterable = [3, 5, 7];
iterable.foo = 'hello';
for (const i in iterable) {
  console.log(i); // logs 0, 1, 2, "foo", "arrCustom", "objCustom"
}
for (const i in iterable) {
  if (iterable.hasOwnProperty(i)) {
    console.log(i); // logs 0, 1, 2, "foo"
  }
}
for (const i of iterable) {
  console.log(i); // logs 3, 5, 7
}
					

Let us look into the above code step by step.

Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};
const iterable = [3, 5, 7];
iterable.foo = 'hello';
					

Every object will inherit the objCustom property and every object that is an Array will inherit the arrCustom property since these properties have been added to Object.prototype and Array.prototype , respectively. The object iterable inherits the properties objCustom and arrCustom because of inheritance and the prototype chain .

for (const i in iterable) {
  console.log(i); // logs 0, 1, 2, "foo", "arrCustom", "objCustom"
}
					

This loop logs only enumerable properties of the iterable object, in arbitrary order. It doesn't log array elements 3 , 5 , 7 or hello because those are not enumerable properties, in fact they are not properties at all, they are values . It logs array indexes arrCustom and objCustom , which are. If you're not sure why these properties are iterated over, there's a more thorough explanation of how array iteration and for...in work.

for (const i in iterable) {
  if (iterable.hasOwnProperty(i)) {
    console.log(i); // logs 0, 1, 2, "foo"
  }
}
					

This loop is similar to the first one, but it uses hasOwnProperty() to check if the found enumerable property is the object's own, i.e. not inherited. If it is, the property is logged. Properties 0 , 1 , 2 and foo are logged because they are own properties ( not inherited ). Properties arrCustom and objCustom are not logged because they are inherited .

for (const i of iterable) {
  console.log(i); // logs 3, 5, 7
}
					

This loop iterates and logs values that iterable , as an iterable object , defines to be iterated over. The object's elements 3 , 5 , 7 are shown, but none of the object's properties .

规范

规范
ECMAScript (ECMA-262)
The definition of 'for...of statement' 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
for...of Chrome 38 Edge 12 Firefox 13
注意事项
13
注意事项
注意事项 Prior to Firefox 51, using the for...of loop construct with the const keyword threw a SyntaxError ("missing = in const declaration").
IE 不支持 No Opera 25 Safari 7 WebView Android 38 Chrome Android 38 Firefox Android 14
注意事项
14
注意事项
注意事项 Prior to Firefox 51, using the for...of loop construct with the const keyword threw a SyntaxError ("missing = in const declaration").
Opera Android 25 Safari iOS 7 Samsung Internet Android 3.0 nodejs 0.12
async iterators Chrome 63 Edge 12 Firefox 57 IE 不支持 No Opera 50 Safari 7 WebView Android 63 Chrome Android 63 Firefox Android 57 Opera Android 46 Safari iOS 7 Samsung Internet Android 8.0 nodejs 10.0.0
Closing iterators Chrome 51 Edge 14 Firefox 53 IE 不支持 No Opera 38 Safari 7 WebView Android 51 Chrome Android 51 Firefox Android 53 Opera Android 41 Safari iOS 7 Samsung Internet Android 5.0 nodejs 6.5.0

图例

完整支持
完整支持
不支持
不支持
见实现注意事项。
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
    1. 介绍
    2. 语法和类型
    3. 控制流程和错误处理
    4. 循环和迭代
    5. 函数
    6. 表达式和运算符
    7. 数字和日期
    8. 文本格式
    9. 正则表达式
    10. Indexed collections
    11. Keyed collections
    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
    1. Client-side JavaScript frameworks
    2. 客户端侧 Web API
    3. 重新介绍 JavaScript
    4. JavaScript 数据结构
    5. 相等比较和相同
    6. 闭包
  6. 高级
    1. 继承和原型链
    2. 严格模式
    3. JavaScript 类型数组
    4. 内存管理
    5. 并发模型和事件循环
  7. 参考:
  8. 内置对象
    1. AggregateError
    2. Array
    3. ArrayBuffer
    4. AsyncFunction
    5. Atomics
    6. BigInt
    7. BigInt64Array
    8. BigUint64Array
    9. Boolean
    10. DataView
    11. Date
    12. Error
    13. EvalError
    14. FinalizationRegistry
    15. Float32Array
    16. Float64Array
    17. Function
    18. Generator
    19. GeneratorFunction
    20. Infinity
    21. Int16Array
    22. Int32Array
    23. Int8Array
    24. InternalError
    25. Intl
    26. JSON
    27. Map
    28. Math
    29. NaN
    30. Number
    31. Object
    32. Promise
    33. Proxy
    34. RangeError
    35. ReferenceError
    36. Reflect
    37. RegExp
    38. Set
    39. SharedArrayBuffer
    40. String
    41. Symbol
    42. SyntaxError
    43. TypeError
    44. TypedArray
    45. URIError
    46. Uint16Array
    47. Uint32Array
    48. Uint8Array
    49. Uint8ClampedArray
    50. WeakMap
    51. WeakRef
    52. WeakSet
    53. WebAssembly
    54. decodeURI()
    55. decodeURIComponent()
    56. encodeURI()
    57. encodeURIComponent()
    58. escape()
    59. eval()
    60. globalThis
    61. isFinite()
    62. isNaN()
    63. null
    64. parseFloat()
    65. parseInt()
    66. undefined
    67. unescape()
    68. uneval()
  9. 表达式 & 运算符
    1. Addition (+)
    2. Addition assignment (+=)
    3. Assignment (=)
    4. Bitwise AND (&)
    5. Bitwise AND assignment (&=)
    6. Bitwise NOT (~)
    7. Bitwise OR (|)
    8. Bitwise OR assignment (|=)
    9. Bitwise XOR (^)
    10. Bitwise XOR assignment (^=)
    11. Comma operator (,)
    12. 条件 (三元) 运算符
    13. Decrement (--)
    14. Destructuring assignment
    15. Division (/)
    16. Division assignment (/=)
    17. Equality (==)
    18. Exponentiation (**)
    19. Exponentiation assignment (**=)
    20. Function expression
    21. Greater than (>)
    22. Greater than or equal (>=)
    23. Grouping operator ( )
    24. Increment (++)
    25. Inequality (!=)
    26. Left shift (<<)
    27. Left shift assignment (<<=)
    28. Less than (<)
    29. Less than or equal (<=)
    30. Logical AND (&&)
    31. Logical AND assignment (&&=)
    32. Logical NOT (!)
    33. Logical OR (||)
    34. Logical OR assignment (||=)
    35. Logical nullish assignment (??=)
    36. Multiplication (*)
    37. Multiplication assignment (*=)
    38. Nullish coalescing operator (??)
    39. Object initializer
    40. 运算符优先级
    41. Optional chaining (?.)
    42. Pipeline operator (|>)
    43. 特性访问器
    44. Remainder (%)
    45. Remainder assignment (%=)
    46. Right shift (>>)
    47. Right shift assignment (>>=)
    48. Spread syntax (...)
    49. Strict equality (===)
    50. Strict inequality (!==)
    51. Subtraction (-)
    52. Subtraction assignment (-=)
    53. Unary negation (-)
    54. Unary plus (+)
    55. Unsigned right shift (>>>)
    56. Unsigned right shift assignment (>>>=)
    57. 异步函数表达式
    58. await
    59. class expression
    60. delete operator
    61. function* 表达式
    62. in operator
    63. instanceof
    64. new operator
    65. new.target
    66. super
    67. this
    68. typeof
    69. void 运算符
    70. yield
    71. yield*
  10. 语句 & 声明
    1. async function
    2. block
    3. break
    4. class
    5. const
    6. continue
    7. debugger
    8. do...while
    9. empty
    10. export
    11. for
    12. for await...of
    13. for...in
    14. for...of
    15. 函数声明
    16. function*
    17. if...else
    18. import
    19. import.meta
    20. label
    21. let
    22. return
    23. switch
    24. throw
    25. try...catch
    26. var
    27. while
    28. with
  11. 函数
    1. 箭头函数表达式
    2. 默认参数
    3. 方法定义
    4. 其余参数
    5. 自变量对象
    6. getter
    7. setter
    1. Private class fields
    2. Public class fields
    3. 构造函数
    4. extends
    5. static
  12. 错误
    1. Error: Permission denied to access property "x"
    2. InternalError: too much recursion
    3. RangeError: argument is not a valid code point
    4. RangeError: invalid array length
    5. RangeError: invalid date
    6. RangeError: precision is out of range
    7. RangeError: radix must be an integer
    8. RangeError: repeat count must be less than infinity
    9. RangeError: repeat count must be non-negative
    10. ReferenceError: "x" is not defined
    11. ReferenceError: assignment to undeclared variable "x"
    12. ReferenceError: can't access lexical declaration`X' before initialization
    13. ReferenceError: deprecated caller or arguments usage
    14. ReferenceError: invalid assignment left-hand side
    15. ReferenceError: reference to undefined property "x"
    16. SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
    17. SyntaxError: "use strict" not allowed in function with non-simple parameters
    18. SyntaxError: "x" is a reserved identifier
    19. SyntaxError: JSON.parse: bad parsing
    20. SyntaxError: Malformed formal parameter
    21. SyntaxError: Unexpected token
    22. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    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
    25. SyntaxError: for-in loop head declarations may not have initializers
    26. SyntaxError: function statement requires a name
    27. SyntaxError: identifier starts immediately after numeric literal
    28. SyntaxError: illegal character
    29. SyntaxError: invalid regular expression flag "x"
    30. SyntaxError: missing ) after argument list
    31. SyntaxError: missing ) after condition
    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
    46. TypeError: "x" is (not) "y"
    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