mock.js

'use strict';

/**
 * Wraps the passed function to track its calls.
 * 
 * All calls are stored in an array on a calls property.
 * Every call is an object with a context and args property.
 */
function trackCalls(fn) {
  const calls = [];
 
  const wrap = function() {
    calls.push({
      args: arguments,
      context: this,
    });
    
    return fn.apply(this, arguments);
  };
  
  wrap.calls = calls;
  
  return wrap;
}

function createMockFunction() {
  let defaultReturnValue;
  const returnValues = [];
  
  const mock = trackCalls(() =>
    (
      returnValues.find(returnValue =>
        arguments.length === returnValue.args.length &&
        returnValue.args.every((arg, index) => arg === arguments[index])
      ) || defaultReturnValue
    ).returnValue;
  );
  
  mock.returns = function (args, returnValue) {
    if (arguments.length === 1) {
      defaultReturnValue = {returnValue: returnValue};
    } else {
      returnValues.push({
        args: args,
        returnValue: returnValue,
      });
    }
    
    return mock;
  };
  
  return mock;
}