Oliver Jumpertz

Posted on Jun 11, 2021 • Edited on Nov 3, 2022 • Originally published at oliverjumpertz.com

JavaScript Quick Tip: Quickly Filter Out All Falsy Values From An Array

#javascript #typescript

Working with arrays in a functional way has mostly become the default when working with JavaScript these days. Why should you use a traditional imperative loop, like for, for..of, while, do..while, etc., when you can use map, filter, and forEach?

These functional methods have one caveat, though: You can never throw from them without aborting the whole pipeline.

someArray.map((value) => {
  if (someConditionMet) {
    throw new Error('...'); // this is not the best idea...
  }
  // ...
  return someValidValue;
});

So what do you do then? Well, you can return null to mark that you have an invalid result.

someArray.map((value) => {
  if (someConditionMet) {
    return null; // now the pipeline can continue
  }
  // ...
  return someValidValue;
});

That's fine. If you don't want your pipeline to abort, you can continue using null values as a marker for "this didn't work out".

But what if you want to get rid of those values afterward? Perhaps something like this?

someArray.map((value) => {
  if (someConditionMet) {
    return null; // now the pipeline can continue
  }
  // ...
  return someValidValue;
}).filter((value) => value);

This leaves you only with valid values, which is perfectly fine, but we can make this even shorter.

The Code

JavaScript has first-class functions. You can pass any function reference to any other function that expects a function as this particular argument. And the Boolean constructor is actually the function responsible to define truthy and falsy.

const array = [1, null, undefined, 0, 2, "", 4];

const result = array.filter(Boolean);

When the filter step of this pipeline has run, you only have all truth values left, and can continue working with them without having to handle special cases like null or undefined.

The Whole Tip As An Image

If you like visual content more, or if you want to store it for later, I put all this into one image for you. I hope you like it!

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Top comments (4)

Amin • Jun 12 '21

Hi Oliver and thanks for your interesting well written article!

In some of my project where I need it, I like to create a filterMap function which allows me to map values to something else, just like we would with the map and get only truthy values in the JavaScript sense.

const filterMap = (callback, items) => {
  return items.reduce((oldItems, item) => {
    const newItem = callback(item);

    if (newItem) {
      return [...oldItems, newItem];
    }

    return oldItems;
  }, []);
};

const getElementById = (identifier) => {
  if (Math.random() > 0.5) {
    return `Element#${identifier}`;
  }

  return null;
};

const identifiers = [
  "paragraph",
  "title",
  "users",
  "answers"
];

const elements = filterMap(getElementById, identifiers);

console.log(elements);
// [ 'Element#paragraph', 'Element#users', 'Element#answers' ]

The pro of using reduce instead of map combined with filter is mainly performance since the reduce call will do one pass where the map and filter will do it in two passes.

Often, this is a recurring pattern and when I see that I am doing a mapping and a filtering, I know that I can do it in one reduce.