The moment I realized forEach() does not return anything.

Javascript ES5's forEach() method and I had been getting along quite well until last week when I was trying to implement it inside this one simple function that takes in two arrays and decides whether they contain common items.

One of my two forEach() loops refused to do its job

So there I had two test arrays to begin with:

const animals = ["lion", "dog", "kangaroo"];
const pets = ["hamster", "parrot", "dog", "cat"];

haveCommonItems(animals, pets); // should return "true"

Apparently the two inputs both contain "dog" so my function on being called should be able to return true in this case (and false in an hypothetically opposite one). For the sake of time complexity optimization, I decided to give it two independent loops (instead of nested ones), using Javascript ES5's Array.prototype.forEach() method:

const haveCommonItems = (array1, array2) => {
  array1.forEach(
    // converts array1 items into properties
    // returns an object that has those properties
  );

  array2.forEach(
    // checks if array2 items equal to properties on object created from first loop
    // returns a boolean
  );
}

My first forEach() loop does its job by yielding the expected object, like so:

const haveCommonItems = (array1) => {
  let obj = {};
  array1.forEach(item => !obj[item] ? obj[item] = true : null);
  console.log(obj);
};

haveCommonItems(animals); // logs {lion: true, elephant: true, dog: true, kangaroo: true}

Things started to confuse me as my second forEach() came along. On running the function I was expecting to get back true which never happened; instead I got a false:

const haveCommonItems = (array1, array2) => {
  let obj = {};
  array1.forEach(item => obj[item] ? null : obj[item] = true);

  array2.forEach(item => {
    if (obj[item]) {
      return true;
    }
  });

  return false;
};

haveCommonItems(animals, pets); // returns "false" (me: "c'mon gimme a break!")

Apparently it seemed like either my array2.forEach() never actually looped through the array itself, or it did but it was just being selfish and not giving back anything.

I was right about the latter.

What actually was going on inside of that so-called loop?

I did a little research by jumping into the MDN web docs only to figure out this one major feature that I had paid almost no attention to before:

forEach() always returns the value undefined and is not chainable.

and

There is no way to stop or break a forEach() loop other than by throwing an exception.

It turned out I had been under the wrong impression that forEach() was there to completely take the job over from my old friend for() loop. In fact, forEach() takes in a callback function that does return a value, but only within the scope created by forEach(). In other words, the return statement inside that callback never brought the value out of its scope or exited the function; it only returned that value to forEach() which then continued to traverse the rest of the array before returning undefined to its boss haveCommonItems().

My function haveCommonItems() at this point (on getting that undefined value) had no idea what happened inside that forEach() (poor little guy!), so it continued on to reach its own return statement and gave me a false.

So yeah, while that AHA moment still lasted, I came up with somewhat a solution for it:

const haveCommonItems = (array1, array2) => {
  let obj = {};
  array1.forEach(item => obj[item] ? null : obj[item] = true);

  let found = false;
  array2.forEach(item => {
    if (obj[item]) {
      found = true;
    }
  });
  return found;
};

haveCommonItems(animals, pets); // returns "true" (me: "you better do!")

Javascript ES5's forEach() method, although always return undefined, does execute side effects. For that reason, it is able to set the value of found to true once the item is equal to any of the obj properties. (Notice that forEach() continues to loop till the end of the array because it can't be stopped or broken unless there is an exception being thrown in).

So here I am, jotting this down hoping not to be fooled by a forEach() loop ever again.

Updated

Thanks to awesome recommendations by my dev fellows below I have levelled up my code which goes like so:

const haveCommonItems = (array1, array2) => {
    const array1set = new Set(array1);
    return array2.some(item => array1set.has(item));

haveCommonItems(animals, pets); // returns "true"
}

Thanks again guys!