function A () {
this.abc = 44;
}
A.prototype.getAbc = function (){
return this.abc;
};
function B() {
}
B.prototype = new A(); // B通过A的实例完成了继承，形成了原型链(B的原型就是A的实例)
var b = new B();
b.getAbc(); 

function Person () { 
}
Person.prototype = {
friends = ["a","b"]; 
}
var person1 = new Person();
var person2 = new Person();
person1.friends.push("c");
console.log(person1.friends); // "a","b","c"
console.log(person2.friends); //"a","b","c" 

function A () {
this.numbers = [1,2,3];
}
function B() {
}
B.prototype = new A();
var b = new B();
var a = new A();
b.numbers.push(4); 
b.numbers; //1234
var b2 = new B(); 
b2.numbers; //1234
a.numbers; //123 

function A (light) {
this.light1 = light;
};
function B (light) {
this.light = light;
};
//给B赋值的同时，想给A赋值，无法实现
B.prototype = new A();
var C = new B(123);
console.log(C.light);
console.log(C.light1); 

function A (light) {
this.light1 = light;
};
function B (light) {
this.light = light;
A.call(this,light);//手动调用A的构造方法
};
//给B赋值的同时，给A赋值
B.prototype = new A();
var C = new B(123);
console.log(C.light);
console.log(C.light1);