在MDN文章《面向对象Javascript介绍》中关于继承的部分,我注意到他们设置了prototype.constructor:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
这有什么重要的目的吗?可以省略吗?
在MDN文章《面向对象Javascript介绍》中关于继承的部分,我注意到他们设置了prototype.constructor:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
这有什么重要的目的吗?可以省略吗?
当前回答
它并不总是必要的,但它确实有它的用途。假设我们想在基类Person上创建一个复制方法。是这样的:
// define the Person Class
function Person(name) {
this.name = name;
}
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new this.constructor(this.name);
};
// define the Student class
function Student(name) {
Person.call(this, name);
}
// inherit Person
Student.prototype = Object.create(Person.prototype);
现在,当我们创建一个新的Student并复制它时会发生什么?
var student1 = new Student("trinth");
console.log(student1.copy() instanceof Student); // => false
副本不是Student的实例。这是因为(如果没有显式检查),我们将无法从“基”类返回Student副本。我们只能返回Person。然而,如果我们重置了构造函数:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
...然后一切都按照预期进行:
var student1 = new Student("trinth");
console.log(student1.copy() instanceof Student); // => true
其他回答
编辑,其实我错了。注释掉一行并不会改变它的行为。(我测试过)
是的,这是必要的。当你这样做时
Student.prototype = new Person();
student。prototype。constructor变成Person。因此,调用Student()将返回Person创建的对象。如果你这样做了
Student.prototype.constructor = Student;
Student.prototype.constructor被重置回Student。现在,当您调用Student()时,它执行Student,它调用父构造函数parent(),它返回正确继承的对象。如果你在调用Student.prototype.constructor之前没有重置它,你将得到一个没有在Student()中设置的任何属性的对象。
当你需要toString的替代而不需要monkeypatching时,它是必要的:
//Local foo = []; foo.toUpperCase = String(foo).toUpperCase; foo.push("a"); foo.toUpperCase(); //Global foo = []; window.toUpperCase = function (obj) {return String(obj).toUpperCase();} foo.push("a"); toUpperCase(foo); //Prototype foo = []; Array.prototype.toUpperCase = String.prototype.toUpperCase; foo.push("a"); foo.toUpperCase(); //toString alternative via Prototype constructor foo = []; Array.prototype.constructor = String.prototype.toUpperCase; foo.push("a,b"); foo.constructor(); //toString override var foo = []; foo.push("a"); var bar = String(foo); foo.toString = function() { return bar.toUpperCase(); } foo.toString(); //Object prototype as a function Math.prototype = function(char){return Math.prototype[char]}; Math.prototype.constructor = function() { var i = 0, unicode = {}, zero_padding = "0000", max = 9999; while (i < max) { Math.prototype[String.fromCharCode(parseInt(i, 16))] = ("u" + zero_padding + i).substr(-4); i = i + 1; } } Math.prototype.constructor(); console.log(Math.prototype("a") ); console.log(Math.prototype["a"] ); console.log(Math.prototype("a") === Math.prototype["a"]);
下面是MDN的一个例子,我发现它对理解它的用法很有帮助。
在JavaScript中,我们有返回AsyncFunction对象的async函数。AsyncFunction不是一个全局对象,但可以通过使用构造函数属性检索它并利用它。
function resolveAfter2Seconds(x) {
return new Promise(resolve => {
setTimeout(() => {
resolve(x);
}, 2000);
});
}
// AsyncFunction constructor
var AsyncFunction = Object.getPrototypeOf(async function(){}).constructor
var a = new AsyncFunction('a',
'b',
'return await resolveAfter2Seconds(a) + await resolveAfter2Seconds(b);');
a(10, 20).then(v => {
console.log(v); // prints 30 after 4 seconds
});
我不同意。不需要设置原型。取完全相同的代码,但去掉原型。构造函数。有什么变化吗?不。现在,进行以下更改:
Person = function () {
this.favoriteColor = 'black';
}
Student = function () {
Person.call(this);
this.favoriteColor = 'blue';
}
在测试代码的末尾……
alert(student1.favoriteColor);
颜色是蓝色的。
对原型的更改。构造函数,根据我的经验,没有做太多,除非你做非常具体,非常复杂的事情,这可能不是一个好的实践:)
Edit: After poking around the web for a bit and doing some experimentation, it looks like people set the constructor so that it 'looks' like the thing that is being constructed with 'new'. I guess I would argue that the problem with this is that javascript is a prototype language - there is no such thing as inheritence. But most programmers come from a background of programming that pushes inheritence as 'the way'. So we come up with all sorts of things to try and make this prototypical language a 'classic' language.. such as extending 'classes'. Really, in the example they gave, a new student is a person - it isn't 'extending' from another student.. the student is all about the person, and whatever the person is the student is as well. Extend the student, and whatever you've extended is a student at heart, but is customized to fit your needs.
克罗克福德有点疯狂和过分热心,但认真阅读他写的一些东西。它会让你从不同的角度看待这个问题。
这有一个巨大的陷阱,如果你写
Student.prototype.constructor = Student;
但如果有一个老师,他的原型也是人,你写
Teacher.prototype.constructor = Teacher;
那么学生构造函数现在是老师!
编辑: 您可以通过确保您已经使用使用Object创建的Person类的新实例设置了Student和Teacher原型来避免这种情况。创建,如Mozilla示例中所示。
Student.prototype = Object.create(Person.prototype);
Teacher.prototype = Object.create(Person.prototype);