1-spy.md

Slides

Repl

ES6 Repl

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Prompt

Testing and assertion libraries like Jasmine, Mocha, Chai, Sinon etc. have a special feature called spies. Spies allow test specs to track how specific functions of interest are used: whether they are called, how many times, what they are called with, what they return, if they throw errors, etc.

For this REACTO problem, implement a spyOn function which does the following:

• Takes a function func as its parameter
• Returns a spy function spy that takes any number of arguments
• spy calls func with the given arguments and returns what func returns
• spy has the following methods:
  • .getCallCount(): 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

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Example

Below is a specific example of how spyOn might work in the wild. Keep in mind that not all functions take only two arguments…

function adder(n1, n2) { return n1 + n2; }

const adderSpy = spyOn( adder );

adderSpy.getCallCount(); // 0

adderSpy(2, 4); // returns 6
adderSpy.getCallCount(); // 1

adderSpy(3, 5); // returns 8
adderSpy.getCallCount(); // 2
adderSpy.wasCalledWith(2); // true
adderSpy.wasCalledWith(0); // false
adderSpy.returned(6); // true
adderSpy.returned(9); // false

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Solution

function spyOn (func) {
  let callCount = 0;
  const calledWith = new Set();
  const returnVals = new Set();

  function spy (...args) {
    const result = func(...args);
    callCount++;
    args.forEach(arg => calledWith.add(arg));
    returnVals.add(result);
    return result;
  }

  spy.getCallCount = function () {
    return callCount;
  };

  spy.wasCalledWith = function (argument) {
    return calledWith.has(argument);
  };

  spy.returned = function (result) {
    return returnVals.has(result);
  };

  return spy;
}

module.exports = spyOn;