ex10.js

/*
SPY

Override a specified method of an object with new functionality while still maintaining all of the old behaviour.

Create a spy that keeps track of how many times a function is called.

## Example

    var spy = Spy(console, 'error')

    console.error('calling console.error')
    console.error('calling console.error')
    console.error('calling console.error')

    console.log(spy.count) // 3

## Arguments

  * target: an object containing the method `method`
  * method: a string with the name of the method on `target` to spy on.

## Conditions

  * Do not use any for/while loops or Array#forEach.
  * Do not create any unnecessary functions e.g. helpers.

## Hint

  * Functions have context, input and output. Make sure you consider the context, input to *and output from* the function you are spying on.

*/

function Spy(target, method) {
  this.count = 0;
  this.originalMethod = target[method];

  target[method] = function() {
    this.originalMethod.apply(target, arguments);
    this.count = this.count + 1;
  }.bind(this);

  return this;
}

var spy = Spy(console, 'error')

console.error('calling console.error')
console.error('calling console.error')
console.error('calling console.error')

console.log(spy.count) // 3