B继承A的写法

1.原型继承：

var A=function(){
this.a=1;
this.b=2;
this.add=function(){
console.log(a+b);
}
}
var B=function(){}
B.prototype=new A();
B.prototype.c=3;
B.prototype.add=function(){
console.log(B.a+B.b+B.c);
}
var b=new B();
console.log(b.a);
console.log(b.b);
console.log(b.c);
b.add();

var A=function(){
this.a=1;
this.b=2;
this.add=function(){
console.log(a+b);
}
}
var B=new A();
B.c=3;
B.add=function () {
console.log(B.a+B.b+B.c);
}
console.log(B.a);
console.log(B.b);
console.log(B.c);
B.add();

function A(){
this.a=1;
this.b=2;
this.add=function(){
console.log(a+b);
}
}
function B(){
A.call(this);
this.c=3;
this.add=function(){
console.log(this.a+this.b+this.c);
}
}
var b=new B();
console.log(b.a);
console.log(b.b);
console.log(b.c);
b.add();

最后

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