js探究prototype，创建参数为数组的函数并为其原型对象添加add、subtract等方法

如果我们想创建一个函数实例（传入参数为数组），为其原型对象添加我们自定义的方法，实现两个数组对象对应的项进行相加、相减等。那我们应该怎么做呢？

我们想得到如下的效果：

var a = new Vector([1,2,3]);
var b = [3,4,5];
var c = [5,6,7,8];

console.log(a.add(b)); // should return [4,6,8]
console.log(a.subtract(b)); // should return [-2,-2,-2]
console.log(a.dot(b)); // should return 1*3+2*4+3*5 = 26
console.log(a.norm()); // should return sqrt(1^2+2^2+3^2)=sqrt(14)
console.log(a.add(c)); // should return false;(两个数组长度不一致)

my answer：

var Vector = function (components) {
  this.value = components;
}

Vector.prototype = {

  add: function (arr) {
      var a = this.value;
      if (a.length == arr.length) {
        var newarr = [];
        for (var i = 0; i < a.length; i++) {
          newarr[i] = a[i]+arr[i]; 
        }
        return newarr;
      } else { 
        return false;
      }
    },
  subtract: function (arr) {
      var a = this.value;
      if (a.length == arr.length) {
        var newarr = [];
        for (var i = 0; i < a.length; i++) {
          newarr[i] = a[i]-arr[i]; 
        }
        return newarr;
      } else { 
        return false;
      }
    },
  dot: function (arr) {
      var a = this.value;
      var b = arr.value;
       if (a.length == arr.length) {
        var sum = 0;
        for (var i = 0; i < a.length; i++) {
           sum += a[i]*arr[i]; 
        }
        return sum;
      } else { 
        return false;
      }
    },
  norm: function () {  
      var a = this.value; 
         var sum = 0;
         for (var i = 0; i < a.length; i++) {
            sum += a[i]*2; 
         }
         return Math.sqrt(sum);
       } 
}