Coding Challenge Practice - Question 39

The task is to implement a spyOn function that works just like the jest.spyOn function.

The boilerplate code:

function spyOn(obj, methodName) {
  // your code here
}

jest.spyOn wraps an existing method on an object, to track how many times it is called, the arguments it is called with, and what it returns.

If the object doesn't exist, or its property isn't a function, it stops immediately

if (!obj || typeof obj[methodName] !== "function") {
  throw new Error("Can't spy on a non-function property");
}

Save the original method

const originalMethod = obj[methodName]

A spy object is created, which keeps all spy data and helper methods.

const spy = {
  calls: [],
  callCount: 0,
  restore() {
    obj[methodName] = originalMethod;
  }
};

Replace the original method, record the call data, and then call the original function. Record the return value.

obj[methodName] = function (...args) {
    spy.callCount++;      
    spy.calls.push(args);

    const result = originalMethod.apply(this, args); 
    spy.lastReturn = result; 

    return result;  

Return the original result.

return result

The final code

function spyOn(obj, methodName) {
  // your code here
  if(!obj || typeof obj[methodName] !== "function") {
    throw new Error("Can't spy on a non-function property")
  }

  const originalMethod = obj[methodName];

  const spy = {
    calls: [],
    callCount: 0,
    restore() {
      obj[methodName] = originalMethod;
    }
  };
  obj[methodName] = function(...args) {
    spy.callCount++;
    spy.calls.push(args);

    const result = originalMethod.apply(this, args);
    spy.lastReturn = result;

    return result;
  }
  return spy;
}

That's all folks!