JavaScript继承
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1)原型链
①原型链示例
function Shape() { this.name = 'shape'; this.toString = function(){ return this.name; }}function TwoDshape () { this.name = '2D shape';}function Triangle (side,height) { this.name = 'Triangle'; this.side = side; this.height = height; this.getArea = function () { return this.side * this.height / 2; }}TwoDshape.prototype = new Shape();Triangle.prototype = new TwoDshape(); //用new新建对象实体,并赋值覆盖该对象的原型TwoDshape.prototype.constructor = TwoDshape;Triangle.prototype.constructor = Triangle; var my = new Triangle(5,10); my.getArea(); //25console.log(my.toString());//继承的方法,具体步骤(遍历my对象属性没有找到,接着查看my.__proto__所指向的对象,即new TwoDshape()创建的实体,//依然没找到,又继续查找该实体的__proto__所指向的对象,即new Shape()所创建的实体,找到toString方法,并在my对象中被调用,this指向my)//通过instanceof操作符,我们可以验证my对象同时是上面三个构造器的实例my instanceof Shape; //truemy instanceof TwoDShape; //truemy instanceof Triangle; //true//我们也可以用其他两个构造器来创建对象,用new TwoDshape()所创建的对象也可以获得继承自Shape()的toString()方法var td = new TwoDshape();td.constructor === TwoDshape; //true;td.toString(); // 2D shapevar s = new Shape();s.constructor === shape; // true;
②将共享属性迁移到原型中去
function Shape(){this.name='shape'}//使用new Shape()新建对象,每个实体都有全新的属性并占用独立空间function Shape(){};Shape.prototype.name='shape';//属性移到原型后,使用new新建对象时,不再含自己独立的这个属性
2)只继承于原型
Triangle.prototype=Shape.prototype;//减少继承方法的查询步骤Triangle.prototype.name='Triangle';//修改子对象原型后父对象原型也随即被改,即再new Shape()新建对象时,新对象name为‘Triangl
②临时构造器——new F()
function Shape() {}Shape.prototype.name = "shape";Shape.prototype.toString = function () { return this.name;}function TwoDshape() {}var F = function () {};F.prototype = Shape.prototype;TwoDshape.prototype = new F();TwoDshape.prototype.constructor = TwoDshape;TwoDshape.prototype.name = '2D shape';function Triangle(side, height) { this.side = side; this.height = height;}var F = function () {};F.prototype = TwoDshape.prototype;Triangle.prototype = new F();Triangle.prototype.constructor = Triangle;Triangle.prototype.name = 'Triangle';Triangle.prototype.getArea = function () { return this.side * this.height / 2;}var my = new Triangle (5,10);alert(my.getArea());//通过这种方法,我们仍然能保持住原型链my._proto_ === Triangle.prototype; //truemy._proto_.constructor === Triangle; //truemy._proto_._proto_ === TwoDshape.prototypr; //truemy._proto_._proto_._proto_.constructor === Shape;_ //true//并且父对象的属性不会被子对象覆盖:var s = new Shape();s.name; // shape//calling toString()"I am a" + new TwoDshape(); //I am a 2D shape
3)uber—子对象访问父对象的方式
function Shape(){}Shape.prototype.name='shape';Shape.prototype.toString=function(){ var const = this.constructor; return const.uber ? this.const.uber.toString() + ',' + this.name : this.name;}function TwoDShape(){var F=function(){}F.prototype=Shape.prototype;TwoDShape.prototype=new F();TwoDShape.prototype.constructor=TwoDShape;TwoDShape.uber=Shape.prototype;TwoDShape.prototype.name='2D shape';function Triangle(side,height){ this.side=side; this.height=height;}var F=function(){}F.prototype=TwoDShape.prototype;Triangle.prototype=new F();Triangle.prototype.constructor=Triangle;Triangle.uber=TwoDShape.prototype;Triangle.prototype.name='triangle';Triangle.prototype.getArea=function(){return this.side*this.height/2};var my=new Triangle(5,10)console.log(my.toString());//shape,2D shape,triangle
4)将继承部分封装成函数
function extend (Child,Parent) { var F = function () {}; F.prototype = Parent.prototype; Child.prototype = new F(); Child.prototype.constructor = Child; Child.uber = Parent.prototype;}extend(TwoDsgape,Shape);extend(Triangle,TwoDshape);
5)属性拷贝
/*属性拷贝执行的是对象原型的逐一拷贝,而非简单的原型链查询。 所以需要特别注意的是: 这种方法仅适用于包含基本数据类型的对象, 所有的对象类型(包括函数和数组)都是不可复制的, 因为他们只支持引用传递 */function extend2(Child,Parent){ var p = Parent.prototype; var c = Child.prototype; for(var i in p){ c[i] = p[i]; } c.uber = p ;}
6)小心处理引用拷贝
var A=function(){},B=function(){};A.prototype.stuff=[1,2,3];A.prototype.name='a';extend2(B,A);//让B继承A使用方法二B.prototype.name+='b';//ab,A.prototype.name依然为a,因为拷贝的是值B.prototype.stuff.push(4);//此时A和B原型上的stuff同时被修改,因为拷贝的是应用B.prototype.stuff=['a','b','c']//如果完全重写事情就不一样了,A为原来,B为新的
7)对象之间的继承(不使用构造器)
function extendCopy (p) { var c = {}; for (var i in p){ c[i] = p[i]; } c.uber = p; return c;}var shape ={ name = 'shape', toString :function () { return this.name; }}var twoDee = extendCopy(shape);twoDee.name = '2D shape';twoDee.toString = function () { return this.uber.toString() + ',' + this.name;}//下面我们让triangle对象继承一个2D图形对象var triangle = extendCopy(twoDee);triangle.name = 'Triangle'triangle.getArea = function () { return this.side * this.height / 2;}//使用triangletriangle.side = 5;triangle.height = 10;triangle.getArea(); //25triangle.toString(); // shape,2D shape,Triangle
8)深拷贝(当遇到对象类型时,再次调用拷贝)
function deepCopy(p,c){ c = c || {}; for (var i in p){ if(p.hasOwnProperty(i)){ if(typeof p[i] === 'object'){ c[i] = Array.isArray(p[i]?[]:{}); deepCopy(p[i],c[i]); }else{ c[i] = p[i] } } } return c;}var parent = { numbers:[1,2,3], letters:['a','b','c'], obj:{ prop : 1 }, bool : true};//我们分别用深拷贝和浅拷贝测试一下,就会发现两者的不同。//在深拷贝中,对对象的numbers属性进行更新不会对原对象产生影响。var mydeep = deepCopy(parent);var myshallow = extendCopy(parent);mydeep.numbers.push(4,5,6);mydeep.numbers //[1,2,3,4,5,6]parent.numbers //[1,2,3]myshallow.numbers.push(10);myshallow.numbers //[1,2,3,10]parent.numbers //[1,2,3,10]
9)object()(用object函数来接受父对象,并返回一个以该对象为原型的新对象)
function object(o){ var n; function F(){} F.prototype=o; n=new F(); n.uber=o; return n;}//这个函数与extendcopy基本相同
10)原型继承与属性拷贝的混合应用
function objectplus(o,stuff){ var n; function F(){} F.prototype=o; n=new F(); n.uber=o; for(var i in stuff){n[i]=stuff[i]} return n;}//两对象o用于继承,stuff用于拷贝方法与属性
11)多重继承(一个对象中有不至一个父对象的继承)
function multi(){ var n={},stuff,j=0,len=arguments.length; for(j=0;i<len;j++){ stuff=arguments[j]; for(var i in stuff){n[i]=stuff[i]} } return n;}//内层循环用于拷贝属性,外层循环用于遍历多个父对象参数,若有相同属性后面替代之前
12) 寄生式继承
13)构造器借用
待续...
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