5kyu_ISpy.js

// 5 kyu - I Spy

// Description:

// In testing, a spy function is one that keeps track of various metadata regarding its invocations. Some examples of properties that a spy might track include:

// Whether it was invoked
// How many times it was invoked
// What arguments it was called with
// What contexts it was called in
// What values it returned
// Whether it threw an error
// For this kata, implement a spyOn function which takes any function func as a parameter and returns a spy for func. The returned spy must be callable in the same manner as the original func, and include the following additional properties/methods:

// .callCount() — returns the number of times spy has been called
// .wasCalledWith(val) – returns true if spy was ever called with val, else returns false.
// .returned(val) — returns true if spy ever returned val, else returns false

function spyOn (func) {
  let call  = [], rVal = [], count = 0;
      
  let spy = function() {
    call = [].concat.call(call, ...arguments);
    var val = func.apply(this, arguments);
    rVal.push(val);
    count+=1;
    return val;
  }
  
  spy.callCount = () => count;
  spy.wasCalledWith = n => call.indexOf(n) > -1;
  spy.returned = n => rVal.indexOf(n) > -1;
  
  return spy;
}