Returning from an anonymous function won't work, even though it's not async

Question

I have the following function:

function filterDesiredURLs(tweet) {
    tweet.entities.urls.forEach((url) => {
        desiredURLs.forEach((regexPattern) => {
            if (regexPattern.test(url['expanded_url'])) {
                console.log('hello, im returning');
                return true;
            }
        })
    })
}

And I'm calling it like this:

console.log(filterDesiredURLs(tweet));

Where tweet is a defined object. I can see that the function is indeed returning because I see the output hello, im returning in the console, but the console.log(filterDesiredURLs(tweet));prints undefined. I would expect this for anonymous functions passed as callbacks for async operations, but this is not async, so the return should work. What's happening?

Answer 1

return doesn't operate across function boundaries. It only returns from the innermost function. To do what you want you probably want filter or find coupled with some:

function filterDesiredURLs(tweet) {
  // First, you were missing a return in the outer function
  // Without a return here, *this* function will return `undefined`
  return tweet.entities.urls.filter(url => {
    // We are using `filter` to reduce the URL list to only
    // URLs that pass our test - this inner function needs to return
    // a boolean (true to include the URL, false to exclude it)
    return desiredURLs.some(regexPattern => {
      // Finally, we use `some` to see if any of the regex patterns match
      // This method returns a boolean value. If the function passed to it ever 
      // returns true, it terminates the loop and returns true
      // Otherwise, it iterates over the entire array and returns false.
      return regexPattern.test(url['expanded_url']);
    });
  });
}