instanceof

instanceof operator tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object. The return value is a boolean value.

句法

object instanceof constructor
					

参数

object
The object to test.
构造函数
Function to test against

描述

instanceof operator tests the presence of constructor.prototype in object 's prototype chain.

// defining constructors
function C() {}
function D() {}
let o = new C()
// true, because: Object.getPrototypeOf(o) === C.prototype
o instanceof C
// false, because D.prototype is nowhere in o's prototype chain
o instanceof D
o instanceof Object           // true, because:
C.prototype instanceof Object // true
C.prototype = {}
let o2 = new C()
o2 instanceof C  // true
// false, because C.prototype is nowhere in
// o's prototype chain anymore
o instanceof C
D.prototype = new C()  // add C to [[Prototype]] linkage of D
let o3 = new D()
o3 instanceof D        // true
o3 instanceof C        // true since C.prototype is now in o3's prototype chain
					

Note that the value of an instanceof test can change based on changes to the prototype property of constructors. It can also be changed by changing an object's prototype using Object.setPrototypeOf . It is also possible using the non-standard __proto__ 特性。

instanceof and multiple context (e.g. frames or windows)

Different scopes have different execution environments. This means that they have different built-ins (different global object, different constructors, etc.). This may result in unexpected results. For instance, [] instanceof window.frames[0].Array will return false , because Array.prototype !== window.frames[0].Array and arrays inherit from the former.

This may not make sense at first, but for scripts dealing with multiple frames or windows, and passing objects from one context to another via functions, this will be a valid and strong issue. For instance, you can securely check if a given object is, in fact, an Array using Array.isArray( myObj )

For example, checking if a 节点 SVGElement in a different context, you can use myNode instanceof myNode.ownerDocument.defaultView.SVGElement .

Note for Mozilla developers:

In code using XPCOM, instanceof has special effect: obj instanceof xpcomInterface (e.g. Components.interfaces.nsIFile ) calls obj .QueryInterface( xpcomInterface ) 并返回 true if QueryInterface succeeded.

A side effect of such call is that you can use xpcomInterface 's properties on obj after a successful instanceof test. Unlike standard JavaScript globals, the test obj instanceof xpcomInterface works as expected, even if obj is from a different scope.

范例

Demonstrating that String and Date are of type Object and exceptional cases

The following code uses instanceof to demonstrate that String and Date objects are also of type Object (they are derived from Object ).

However, objects created with the object literal notation are an exception here: Although the prototype is undefined , instanceof Object 返回 true .

let simpleStr = 'This is a simple string'
let myString  = new String()
let newStr    = new String('String created with constructor')
let myDate    = new Date()
let myObj     = {}
let myNonObj  = Object.create(null)
simpleStr instanceof String  // returns false, string literal is not an object
myString  instanceof String  // returns true
newStr    instanceof String  // returns true
myString  instanceof Object  // returns true
myObj    instanceof Object   // returns true, every object literal has Object.prototype as prototype
({})     instanceof Object   // returns true, same case as above
myNonObj instanceof Object   // returns false, prototype is end of prototype chain (null)
myString instanceof Date     // returns false
myDate instanceof Date      // returns true
myDate instanceof Object    // returns true
myDate instanceof String    // returns false
					

Demonstrating that mycar 是类型 Car and type Object

The following code creates an object type Car and an instance of that object type, mycar instanceof operator demonstrates that the mycar object is of type Car and of type Object .

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
let mycar = new Car('Honda', 'Accord', 1998)
let a = mycar instanceof Car     // returns true
let b = mycar instanceof Object  // returns true
					

Not an instanceof

To test if an object is not an instanceof a specific constructor, you can do

if (!(mycar instanceof Car)) {
  // Do something, like:
  // mycar = new Car(mycar)
}
					

This is really different from:

if (!mycar instanceof Car)
					

This will always be false . ( !mycar will be evaluated before instanceof , so you always try to know if a boolean is an instance of Car ).

规范

规范
ECMAScript (ECMA-262)
The definition of 'Relational Operators' 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
instanceof Chrome 1 Edge 12 Firefox 1 IE 5 Opera 4 Safari 1 WebView Android 1 Chrome Android 18 Firefox Android 4 Opera Android 10.1 Safari iOS 1 Samsung Internet Android 1.0 nodejs 0.1.100

图例

完整支持
完整支持

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
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  4. JavaScript 指南
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    2. 语法和类型
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    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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    23. Grouping operator ( )
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    25. Inequality (!=)
    26. Left shift (<<)
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    28. Less than (<)
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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 (*)
    37. Multiplication assignment (*=)
    38. Nullish coalescing operator (??)
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    53. Unary negation (-)
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  12. 错误
    1. Error: Permission denied to access property "x"
    2. InternalError: too much recursion
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    5. RangeError: invalid date
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    8. RangeError: repeat count must be less than infinity
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    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