Lambda JS #3: Filter

Brazil — PA — Belém — Teatro da Paz. Photo by Socorro Simonetti. Art by Rian Vergara.

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Another common looping scenario is to filter elements from a array. Usually it would be done like this:

Yea, once again you write that boilerplate for loop.

As you may have guessed by the title of this story, JavaScript indeed have a method to handle this boilerplate.

Filter

Just like map, Array#filter will create a new array looping over a source array. The main difference is that it’ll only include a value in the new array if the callback returns true — that is, the callback tests if the element should be added. The callback signature is the same as map’s:

1. Current item value.
2. Current item index.
3. The array being looped on.

Your callback should return either a truthy or a falsy value. If you don’t return anything, undefined is understood as false.

Review

1. Use filter when you need to make a array with values that follow a criteria.
2. The filter callback return a truthy value to add the value to the new array or a falsy to ignore it.

Links

Lambda JS #4: Chaining

Array.prototype.filter()