无法在javascript中访问函数的原型方法

Question

I am going through JS the good parts and come to this example. In the last line I attempt to call upon a method that is defined in the prototype chain of the sum function. I am confused as to why this does not work.

Define the sum function:

var sum = function(){
    var i, sum=0;
    for(i=0;i<arguments.length;i+=1){
        sum += arguments[i];
    }
    return sum;
}

add the method method to the function prototype

Function.prototype.method = function(name, func){
    this.prototype.name = func;
}

use the method method to add an 'add' method to sum

sum.method('add', function(a,b){
    return a + b;
})

The prototype method is clearly defined but I can not access it via the prototype chain:

sum.prototype.add(5,5); //returns 10
sum.add(5,5); //TypeError: sum.add not defined

In the last line why can I not access the method defined in the prototype chain of the sum function?

Answer 1

sum.prototype has nothing to do with the prototype of the function. sum.prototype is the prototype of instances of sum when called as constructor (ie with new ):

var instance = new sum(); // doesn't make sense, I know
Object.getPrototypeOf(instance) === sum.prototype; // true
instance.add // function

The prototype of sum itself can be accessed via Object.getPrototypeOf(sum) and is Function.prototype .

Object.getPrototypeOf(sum) === Function.prototype; // true

Btw, that's exactly why sum.method() works: You are extending sum 's prototype by assigning to Function.prototype.method .

---

I'm not sure what you are trying to do, but extending functions via their prototype chain is probably not the right solution to your problem.

If you want to add a property to sum , then just assign it directly:

sum.add = function() { ... };