轉載自:http://developer.51cto.com/art/200907/134913.htm
constructor屬性始終指向創建當前對象的構造函數。比如下面例子:
// 等價於 var foo = new Array(1, 56, 34, 12);
var arr = [1, 56, 34, 12];
console.log(arr.constructor === Array); // true
// 等價於 var foo = new Function();
var Foo = function() { };
console.log(Foo.constructor === Function); // true
// 由構造函數實例化一個obj對象
var obj = new Foo();
console.log(obj.constructor === Foo); // true
// 將上面兩段代碼合起來,就得到下面的結論
console.log(obj.constructor.constructor === Function); // true
但是當constructor遇到prototype時,有趣的事情就發生了。
我們知道每個函數都有一個默認的屬性prototype,而這個prototype的constructor默認指向這個函數。如下例所示:
function Person(name) {
this.name = name;
};
Person.prototype.getName = function() {
return this.name;
};
var p = new Person("ZhangSan");
console.log(p.constructor === Person); // true
console.log(Person.prototype.constructor === Person); // true
// 將上兩行代碼合併就得到如下結果
console.log(p.constructor.prototype.constructor === Person); // true
當時當我們重新定義函數的prototype時(注意:和上例的區別,這裏不是修改而是覆蓋),constructor屬性的行爲就有點奇怪了,如下示例:
function Person(name) {
this.name = name;
};
Person.prototype = {
getName: function() {
return this.name;
}
};
var p = new Person("ZhangSan");
console.log(p.constructor === Person); // false
console.log(Person.prototype.constructor === Person); // false
console.log(p.constructor.prototype.constructor === Person); // false
爲什麼呢?
原來是因爲覆蓋Person.prototype時,等價於進行如下代碼操作:
Person.prototype = new Object({
getName: function() {
return this.name;
}
});
而constructor屬性始終指向創建自身的構造函數,所以此時Person.prototype.constructor === Object,即是:
function Person(name) {
this.name = name;
};
Person.prototype = {
getName: function() {
return this.name;
}
};
var p = new Person("ZhangSan");
console.log(p.constructor === Object); // true
console.log(Person.prototype.constructor === Object); // true
console.log(p.constructor.prototype.constructor === Object); // true
怎麼修正這種問題呢?方法也很簡單,重新覆蓋Person.prototype.constructor即可:
function Person(name) {
this.name = name;
};
Person.prototype = new Object({
getName: function() {
return this.name;
}
});
Person.prototype.constructor = Person;
var p = new Person("ZhangSan");
console.log(p.constructor === Person); // true
console.log(Person.prototype.constructor === Person); // true
console.log(p.constructor.prototype.constructor === Person); // true