creating _.memoize function that takes in a function as a parameter

Question

I just started learning about memoization today and i am having a hard time wrapping my head around it. I am trying to create a memoize function that stores the result of the parameter function that has been called already and return its value instead of calling it again This is my code:

_.memoize = function(func) {
    var stored = {};

    if (stored.hasOwnProperty('calculated')) {
      return stored['calculated'];
    }

    stored['calculated'] = func;

    return func;
  };

The test cases are:

var spy = sinon.spy(function() { return 'Dummy output'; });
var memoSpy = _.memoize(spy);
memoSpy(10);
expect(spy).to.have.been.calledOnce;
memoSpy(10);
expect(spy).to.have.been.calledOnce;

And:

var spy = sinon.spy(function() { return 'Dummy output'; });
var memoSpy = _.memoize(spy);
memoSpy([1, 2, 3]);
expect(spy).to.have.been.calledOnce;
memoSpy([1, 2, 3]);
expect(spy).to.have.been.calledOnce;

I thought my stored object stores the result of func?

The error i am getting is basically saying that 'spy' should have been called once yet its getting called twice.

Answer 1

The problem is, you are returning the original func. you have to do some 2nd order stuff like

var memoize = function(func) {
  var stored = {};

  return (function() {      
    if (!stored.hasOwnProperty('calculated')) {
      stored['calculated'] = func();
    }

    return stored['calculated'];
  });
};

// To test
var x = function() { console.log('function called'); return 42; }

y = memoize(x)

y()
// => function called
// 42

y()
// 42

Where you basically return a modified/extended function that wraps the original func and adds the memoizing behavior.