instanceof关键字

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instanceof for testing object instance, cannot test primitive type

适用场景

  • it’s safe to say that instanceof is applicable only for objects rather than primitive types.
  • typeof operator tests whether the value belongs to primitive types like “string”, “number”, “boolean”, “object”, “function” or “undefined”.

polyfill实现

根据左值原型指针和右边prototype做比较，如果相等则为true,否则沿原型链向上找，直到Object.prototype.__proto为null

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function myInstanceOf(L, R) {
    let proto = L.__proto__;
    let prototype = R.prototype;
    while (proto) {
        if (proto === prototype) {
            return true;
        }
        proto = proto.__proto__
    }
    return false;
}

var aa = [1];
//aa.__proto__ === Array.prototype true
//Array.prototype.__proto__ === Object.prototype true
//Object.prototype.__proto__ null
console.log(myInstanceOf(aa, Object))