如何检查 Javascript 类是否继承了另一个类（不创建 obj）？
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How to check if a Javascript Class inherits another (without creating an obj)?
提问by Simon
Eg:
例如：
function A(){}
function B(){}
B.prototype = new A();
How can I check if the class B inherits class A?
如何检查B类是否继承了A类？
回答by Nirvana Tikku
Try the following:
请尝试以下操作：
ChildClass.prototype instanceof ParentClass
回答by Denys Séguret
You can test direct inheritance with
您可以测试直接继承
B.prototype.constructor === A
To test indirect inheritance, you may use:
要测试间接继承，您可以使用：
B.prototype instanceof A
(this second solution was first given by Nirvana Tikku)
（这第二个解决方案首先由 Nirvana Tikku 给出）
回答by pery mimon
back to 2017:
check if that work for you
回到 2017 年：
检查这是否适合您
ParentClass.isPrototypeOf(ChildClass)
回答by jlchereau
I do not think Simon meant B.prototype = new A()in his question, because this is certainly not the way to chain prototypes in JavaScript.
我不认为 SimonB.prototype = new A()在他的问题中的意思，因为这肯定不是在 JavaScript 中链接原型的方式。
Assuming B extends A, use Object.prototype.isPrototypeOf.call(A.prototype, B.prototype)
假设 B 扩展 A，使用 Object.prototype.isPrototypeOf.call(A.prototype, B.prototype)
回答by Pacerier
Gotchas:Note that instanceofdoes not work as expected if you use multiple execution contexts/windows. See §§.
问题：请注意，instanceof如果您使用多个执行上下文/窗口，则不会按预期工作。见§§。
Also, per https://johnresig.com/blog/objectgetprototypeof/, this is an alternative implementation that is identical to instanceof:
此外，根据https://johnresig.com/blog/objectgetprototypeof/，这是一个与以下相同的替代实现instanceof：
function f(_, C) { // instanceof Polyfill
while (_ != null) {
if (_ == C.prototype)
return true;
_ = _.__proto__;
}
return false;
}
Modifying it to check the class directly gives us:
修改它以直接检查类给我们：
function f(ChildClass, ParentClass) {
_ = ChildClass.prototype;
while (_ != null) {
if (_ == C.prototype)
return true;
_ = _.__proto__;
}
return false;
}
Sidenote边注
instanceofitself checks if obj.protois f.prototype, thus:
instanceof本身检查是否obj.proto为f.prototype，因此：
function A(){};
A.prototype = Array.prototype;
[]instanceof Array // true
and:
和：
function A(){}
_ = new A();
// then change prototype:
A.prototype = [];
/*false:*/ _ instanceof A
// then change back:
A.prototype = _.__proto__
_ instanceof A //true
and:
和：
function A(){}; function B(){};
B.prototype=Object.prototype;
/*true:*/ new A()instanceof B
If it's not equal, proto is swapped with proto of proto in the check, then proto of proto of proto, and so on. Thus:
如果不相等，则 proto 在检查中与 proto 的 proto 交换，然后是 proto of proto of proto 的 proto，依此类推。因此：
function A(){}; _ = new A()
_.__proto__.__proto__ = Array.prototype
g instanceof Array //true
and:
和：
function A(){}
A.prototype.__proto__ = Array.prototype
g instanceof Array //true
and:
和：
f=()=>{};
f.prototype=Element.prototype
document.documentElement instanceof f //true
document.documentElement.__proto__.__proto__=[];
document.documentElement instanceof f //false
