如何在javascript中从子类调用父方法？

Question

我花了最后几个小时试图找到问题的解决方案，但似乎没有希望。

基本上我需要知道如何从子类调用父方法。 到目前为止，我尝试过的所有内容都会导致无法正常工作或覆盖父方法。

我使用以下代码在javascript中设置OOP：

// SET UP OOP
// surrogate constructor (empty function)
function surrogateCtor() {}

function extend(base, sub) {
    // copy the prototype from the base to setup inheritance
    surrogateCtor.prototype = base.prototype;
    sub.prototype = new surrogateCtor();
    sub.prototype.constructor = sub;
}

// parent class
function ParentObject(name) {
    this.name = name;
}
// parent's methods
ParentObject.prototype = {
    myMethod: function(arg) {
        this.name = arg;
    }
}

// child
function ChildObject(name) {
    // call the parent's constructor
    ParentObject.call(this, name);
    this.myMethod = function(arg) {
        // HOW DO I CALL THE PARENT METHOD HERE?
        // do stuff
    }
}

// setup the prototype chain
extend(ParentObject, ChildObject);

我需要先调用父方法，然后在子类中添加更多内容。

在大多数OOP语言中，就像调用parent.myMethod()一样简单 但我真的无法掌握它是如何在javascript中完成的。

非常感谢任何帮助，谢谢！

Answer 1

以下是它的完成方式：{​​{1}}

或者如果您想在当前实例的上下文中调用它，您可以执行以下操作： ParentClass.prototype.myMethod();

从带有参数的子类调用父方法也是如此： ParentClass.prototype.myMethod.call(this) * 提示：使用ParentClass.prototype.myMethod.call(this, arg1, arg2, ..)代替apply()将参数作为数组传递。

Answer 2

ES6风格允许您使用新功能，例如super关键字。当您使用ES6类语法时，super关键字是关于父类上下文的。作为一个非常简单的例子，结帐：

class Foo {
    static classMethod() {
        return 'hello';
    }
}

class Bar extends Foo {
    static classMethod() {
        return super.classMethod() + ', too';
    }
}
Bar.classMethod(); // 'hello, too'

此外，您可以使用super来调用父构造函数：

class Foo {}

class Bar extends Foo {
    constructor(num) {
        let tmp = num * 2; // OK
        this.num = num; // ReferenceError
        super();
        this.num = num; // OK
    }
}

当然，您可以使用它来访问父类属性super.prop。 所以，使用ES6并感到高兴。

Answer 3

为了做到这一点，你不受限于ES6的Class抽象。通过__proto__属性可以访问父构造函数的原型方法（我很确定会有JS同事抱怨它已经折旧了），它被折旧但同时被发现它实际上是子类需求的基本工具（特别是对于Array子类需求）。因此，虽然__proto__属性仍然可用于我所知道的所有主要JS引擎，但ES6在其上引入了Object.getPrototypeOf()功能。 super()抽象中的Class工具是这个的语法糖。

因此，如果您无法访问父级构造函数的名称，并且不想使用Class抽象，您仍可按以下步骤操作;

function ChildObject(name) {
    // call the parent's constructor
    ParentObject.call(this, name);
    this.myMethod = function(arg) {
    //this.__proto__.__proto__.myMethod.call(this,arg);
    Object.getPrototypeOf(Object.getPrototypeOf(this)).myMethod.call(this,arg);
    }
}

Answer 4

在多继承级别的情况下，此函数可以在其他语言中用作super（）方法。 Here is a demo fiddle，通过一些测试，您可以在方法使用中使用它：call_base(this, 'method_name', arguments);

它利用了最近的ES功能，与旧版浏览器的兼容性并不能保证。在IE11，FF29，CH35中测试。

/**
 * Call super method of the given object and method.
 * This function create a temporary variable called "_call_base_reference",
 * to inspect whole inheritance linage. It will be deleted at the end of inspection.
 *
 * Usage : Inside your method use call_base(this, 'method_name', arguments);
 *
 * @param {object} object The owner object of the method and inheritance linage
 * @param {string} method The name of the super method to find.
 * @param {array} args The calls arguments, basically use the "arguments" special variable.
 * @returns {*} The data returned from the super method.
 */
function call_base(object, method, args) {
    // We get base object, first time it will be passed object,
    // but in case of multiple inheritance, it will be instance of parent objects.
    var base = object.hasOwnProperty('_call_base_reference') ? object._call_base_reference : object,
    // We get matching method, from current object,
    // this is a reference to define super method.
            object_current_method = base[method],
    // Temp object wo receive method definition.
            descriptor = null,
    // We define super function after founding current position.
            is_super = false,
    // Contain output data.
            output = null;
    while (base !== undefined) {
        // Get method info
        descriptor = Object.getOwnPropertyDescriptor(base, method);
        if (descriptor !== undefined) {
            // We search for current object method to define inherited part of chain.
            if (descriptor.value === object_current_method) {
                // Further loops will be considered as inherited function.
                is_super = true;
            }
            // We already have found current object method.
            else if (is_super === true) {
                // We need to pass original object to apply() as first argument,
                // this allow to keep original instance definition along all method
                // inheritance. But we also need to save reference to "base" who
                // contain parent class, it will be used into this function startup
                // to begin at the right chain position.
                object._call_base_reference = base;
                // Apply super method.
                output = descriptor.value.apply(object, args);
                // Property have been used into super function if another
                // call_base() is launched. Reference is not useful anymore.
                delete object._call_base_reference;
                // Job is done.
                return output;
            }
        }
        // Iterate to the next parent inherited.
        base = Object.getPrototypeOf(base);
    }
}

Answer 5

基于Douglas Crockford想法的事情怎么样：

    function Shape(){}

    Shape.prototype.name = 'Shape';

    Shape.prototype.toString = function(){
        return this.constructor.parent
            ? this.constructor.parent.toString() + ',' + this.name
            : this.name;
    };


    function TwoDShape(){}

    var F = function(){};

    F.prototype = Shape.prototype;

    TwoDShape.prototype = new F();

    TwoDShape.prototype.constructor = TwoDShape;

    TwoDShape.parent = Shape.prototype;

    TwoDShape.prototype.name = '2D Shape';


    var my = new TwoDShape();

    console.log(my.toString()); ===> Shape,2D Shape

Answer 6

有一种用于多层原型查找的更轻松，更紧凑的解决方案，但它需要Proxy支持。用法：SUPER(<instance>).<method>(<args>)，例如，假设使用方法A的两个类B extends A和m：SUPER(new B).m()。

function SUPER(instance)
{
    return new Proxy(instance,
    {
        get(target, prop)
        {
            return Object.getPrototypeOf(Object.getPrototypeOf(target))[prop].bind(target);
        }
    });
}

Answer 7

这是子对象使用JavaScript的原型链访问父级属性和方法的一种好方法，并且与Internet Explorer兼容。 JavaScript搜索原型链中的方法，我们希望孩子的原型链看起来像这样：

Child实例-> Child的原型（带有Child方法）-> Parent的原型（带有Parent方法）->对象原型-> null

子方法也可以调用带阴影的父方法，如下面的三个星号***所示。

方法如下：

//Parent constructor
function ParentConstructor(firstName){
    //add parent properties:
    this.parentProperty = firstName;
}

//add 2 Parent methods:
ParentConstructor.prototype.parentMethod = function(argument){
    console.log(
            "Parent says: argument=" + argument +
            ", parentProperty=" + this.parentProperty +
            ", childProperty=" + this.childProperty
    );
};

ParentConstructor.prototype.commonMethod = function(argument){
    console.log("Hello from Parent! argument=" + argument);
};

//Child constructor    
function ChildConstructor(firstName, lastName){
    //first add parent's properties
    ParentConstructor.call(this, firstName);

    //now add child's properties:
    this.childProperty = lastName;
}

//insert Parent's methods into Child's prototype chain
var rCopyParentProto = Object.create(ParentConstructor.prototype);
rCopyParentProto.constructor = ChildConstructor;
ChildConstructor.prototype = rCopyParentProto;

//add 2 Child methods:
ChildConstructor.prototype.childMethod = function(argument){
    console.log(
            "Child says: argument=" + argument +
            ", parentProperty=" + this.parentProperty +
            ", childProperty=" + this.childProperty
    );
};

ChildConstructor.prototype.commonMethod = function(argument){
    console.log("Hello from Child! argument=" + argument);

    // *** call Parent's version of common method
    ParentConstructor.prototype.commonMethod(argument);
};

//create an instance of Child
var child_1 = new ChildConstructor('Albert', 'Einstein');

//call Child method
child_1.childMethod('do child method');

//call Parent method
child_1.parentMethod('do parent method');

//call common method
child_1.commonMethod('do common method');

Answer 8

虽然您可以通过父级的原型来调用父级方法，但是您将需要传递当前的子级实例，以使用call，apply或bind方法。 bind方法将创建一个新函数，因此我不建议您在只考虑一次调用的情况下关注性能。

作为替代方案，您可以替换子方法并将父方法放在实例上，同时调用原始子方法。

function proxy(context, parent){
  var proto = parent.prototype;
  var list = Object.getOwnPropertyNames(proto);
  
  var child = {};
  for(var i=0; i<list.length; i++){
    var key = list[i];

    // Create only when child have similar method name
    if(context[key] !== proto[key]){
      child[key] = context[key];
      context[key] = function(){
        context.super = proto[key];
        return child[key].apply(context, arguments);
      }
    }
  }
}

// ========= The usage would be like this ==========

class Parent {
  first = "Home";

  constructor(){
    console.log('Parent created');
  }

  add(arg){
    return this.first + ", Parent "+arg;
  }
}

class Child extends Parent{
  constructor(b){
    super();
    proxy(this, Parent);
    console.log('Child created');
  }

  // Comment this to call method from parent only
  add(arg){
    return this.super(arg) + ", Child "+arg;
  }
}

var family = new Child();
console.log(family.add('B'));

Answer 9

使用经典 js 提供更灵活的答案。

您定义“_parent = A.prototype;”在子类中，您可以使用 apply 调用父类的方法：

class A{
    _msg='A';
    _msgOnlyA=' great from A';
    constructor(){
    }
    hello(){
        console.log('hello '+this._msg+', '+this._msgOnlyA);
    }
};
class B extends A{
    _parent = A.prototype;
    _msg='B';
    constructor(){
        super();
    }
    hello(){
        this._parent.hello.apply(this, arguments);
        console.log('hello '+this._msg);
    }
};
var b = new B();
b.hello();