instanceof

¥Syntax

object instanceof constructor

¥Parameters

object

要测试的对象。

constructor

要测试的构造函数。

¥Exceptions

TypeError

如果 constructor 不是对象，则抛出该异常。如果 constructor 没有 @@hasInstance 方法，那么它也必须是一个函数。

¥Description

instanceof 运算符测试 constructor.prototype 在 object 原型链中的存在。这通常（尽管 不总是）意味着 object 是用 constructor 构建的。

¥The instanceof operator tests the presence of constructor.prototype in object's prototype chain. This usually (though not always) means object was constructed with constructor.

// defining constructors
function C() {}
function D() {}

const 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

// Re-assign `constructor.prototype`: you should
// rarely do this in practice.
C.prototype = {};
const 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
const o3 = new D();
o3 instanceof D; // true
o3 instanceof C; // true since C.prototype is now in o3's prototype chain

请注意，如果在创建对象后重新分配 constructor.prototype（通常不鼓励这样做），则 instanceof 测试的值可能会更改。也可以通过使用 Object.setPrototypeOf 改变 object 的原型来改变。

¥Note that the value of an instanceof test can change if constructor.prototype is re-assigned after creating the object (which is usually discouraged). It can also be changed by changing object's prototype using Object.setPrototypeOf.

类的行为方式相同，因为类也具有 prototype 属性。

¥Classes behave in the same way, because classes also have the prototype property.

class A {}
class B extends A {}

const o1 = new A();
// true, because Object.getPrototypeOf(o1) === A.prototype
o1 instanceof A;
// false, because B.prototype is nowhere in o1's prototype chain
o1 instanceof B;

const o2 = new B();
// true, because Object.getPrototypeOf(Object.getPrototypeOf(o2)) === A.prototype
o2 instanceof A;
// true, because Object.getPrototypeOf(o2) === B.prototype
o2 instanceof B;

对于 绑定函数，instanceof 在目标函数上查找 prototype 属性，因为绑定函数没有 prototype。

¥For bound functions, instanceof looks up for the prototype property on the target function, since bound functions don't have prototype.

class Base {}
const BoundBase = Base.bind(null, 1, 2);
console.log(new Base() instanceof BoundBase); // true

¥instanceof and @@hasInstance

如果 constructor 有 Symbol.hasInstance 方法，则优先调用该方法，以 object 作为其唯一参数，以 constructor 作为 this。

¥If constructor has a Symbol.hasInstance method, the method will be called in priority, with object as its only argument and constructor as this.

// This class allows plain objects to be disguised as this class's instance,
// as long as the object has a particular flag as its property.
class Forgeable {
  static isInstanceFlag = Symbol("isInstanceFlag");

  static [Symbol.hasInstance](obj) {
    return Forgeable.isInstanceFlag in obj;
  }
}

const obj = { [Forgeable.isInstanceFlag]: true };
console.log(obj instanceof Forgeable); // true

因为默认情况下所有函数都继承自 Function.prototype，所以大多数时候，Function.prototype[@@hasInstance] 方法指定了当右侧是函数时 instanceof 的行为。instanceof 的具体算法请参见 Symbol.hasInstance 页面。

¥Because all functions inherit from Function.prototype by default, most of the time, the Function.prototype[@@hasInstance] method specifies the behavior of instanceof when the right-hand side is a function. See the Symbol.hasInstance page for the exact algorithm of instanceof.

¥instanceof and multiple realms

JavaScript 执行环境（窗口、框架等）各有其自己的字段。这意味着它们有不同的内置函数（不同的全局对象、不同的构造函数等）。这可能会导致意想不到的结果。例如，[] instanceof window.frames[0].Array 将返回 false，因为当前字段中的 Array.prototype !== window.frames[0].Array.prototype 和数组继承自前者。

¥JavaScript execution environments (windows, frames, etc.) are each in their own realm. 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.prototype and arrays in the current realm inherit from the former.

一开始这可能没有意义，但对于处理多个框架或窗口的脚本，以及通过函数将对象从一个上下文传递到另一个上下文，这将是一个有效且强大的问题。例如，你可以使用 Array.isArray() 安全地检查给定对象是否实际上是一个数组，而忽略它来自哪个字段。

¥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(), neglecting which realm it comes from.

例如，要检查 Node 在不同上下文中是否是 SVGElement，可以使用 myNode instanceof myNode.ownerDocument.defaultView.SVGElement。

¥For example, to check if a Node is an SVGElement in a different context, you can use myNode instanceof myNode.ownerDocument.defaultView.SVGElement.

¥Examples

¥Using instanceof with String

以下示例显示了 instanceof 与 String 对象的行为。

¥The following example shows the behavior of instanceof with String objects.

const literalString = "This is a literal string";
const stringObject = new String("String created with constructor");

literalString instanceof String; // false, string primitive is not a String
stringObject instanceof String; // true

literalString instanceof Object; // false, string primitive is not an Object
stringObject instanceof Object; // true

stringObject instanceof Date; // false

¥Using instanceof with Date

以下示例显示了 instanceof 与 Date 对象的行为。

¥The following example shows the behavior of instanceof with Date objects.

const myDate = new Date();

myDate instanceof Date; // true
myDate instanceof Object; // true
myDate instanceof String; // false

¥Objects created using Object.create()

以下示例显示了 instanceof 与使用 Object.create() 创建的对象的行为。

¥The following example shows the behavior of instanceof with objects created using Object.create().

function Shape() {}

function Rectangle() {
  Shape.call(this); // call super constructor.
}

Rectangle.prototype = Object.create(Shape.prototype);

Rectangle.prototype.constructor = Rectangle;

const rect = new Rectangle();

rect instanceof Object; // true
rect instanceof Shape; // true
rect instanceof Rectangle; // true
rect instanceof String; // false

const literalObject = {};
const nullObject = Object.create(null);
nullObject.name = "My object";

literalObject instanceof Object; // true, every object literal has Object.prototype as prototype
({}) instanceof Object; // true, same case as above
nullObject instanceof Object; // false, prototype is end of prototype chain (null)

¥Demonstrating that mycar is of type Car and type Object

以下代码创建对象类型 Car 和该对象类型 mycar 的实例。instanceof 运算符表明 mycar 对象的类型为 Car 且类型为 Object。

¥The following code creates an object type Car and an instance of that object type, mycar. The 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;
}
const mycar = new Car("Honda", "Accord", 1998);
const a = mycar instanceof Car; // returns true
const b = mycar instanceof Object; // returns true

¥Not an instanceof

要测试对象是否不是 instanceof 特定构造函数，你可以执行以下操作：

¥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)
}

这确实不同于：

¥This is really different from:

if (!mycar instanceof Car) {
  // unreachable code
}

这将永远是 false。（!mycar 将在 instanceof 之前计算，因此你总是尝试知道布尔值是否是 Car 的实例）。

¥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).

¥Overriding the behavior of instanceof

使用 instanceof 的一个常见陷阱是相信，如果 x instanceof C，那么 x 是使用 C 作为构造函数创建的。这是不正确的，因为 x 可以直接用 C.prototype 作为其原型来赋值。在这种情况下，如果你的代码从 x 读取 私有字段 of C，它仍然会失败：

¥A common pitfall of using instanceof is believing that, if x instanceof C, then x was created using C as constructor. This is not true, because x could be directly assigned with C.prototype as its prototype. In this case, if your code reads private fields of C from x, it would still fail:

class C {
  #value = "foo";
  static getValue(x) {
    return x.#value;
  }
}

const x = { __proto__: C.prototype };

if (x instanceof C) {
  console.log(C.getValue(x)); // TypeError: Cannot read private member #value from an object whose class did not declare it
}

为了避免这种情况，你可以通过向 C 添加 Symbol.hasInstance 方法来覆盖 instanceof 的行为，以便它对 in 进行品牌检查：

¥To avoid this, you can override the behavior of instanceof by adding a Symbol.hasInstance method to C, so that it does a branded check with in:

class C {
  #value = "foo";

  static [Symbol.hasInstance](x) {
    return #value in x;
  }

  static getValue(x) {
    return x.#value;
  }
}

const x = { __proto__: C.prototype };

if (x instanceof C) {
  // Doesn't run, because x is not a C
  console.log(C.getValue(x));
}

请注意，你可能希望将此行为限制为当前类；否则，可能会导致子类出现误报：

¥Note that you may want to limit this behavior to the current class; otherwise, it could lead to false positives for subclasses:

class D extends C {}
console.log(new C() instanceof D); // true; because D inherits @@hasInstance from C

你可以通过检查 this 是否是当前构造函数来做到这一点：

¥You could do this by checking that this is the current constructor:

class C {
  #value = "foo";

  static [Symbol.hasInstance](x) {
    return this === C && #value in x;
  }
}

class D extends C {}
console.log(new C() instanceof D); // false
console.log(new C() instanceof C); // true
console.log({ __proto__: C.prototype } instanceof C); // false

¥See also

• typeof
• Symbol.hasInstance
• Object.prototype.isPrototypeOf