what is array filter() in JavaScript

Brian Munoz

filter() run a function on every element in an array and keeps the element if the function returned a truthy value otherwise remove them. The original array isn’t modified.

The function passed into filter() takes in these parameters:

current element
current element index
array that filter() is being called on

An example could be removing duplicates.

JavaScript

const arrayWithDuplicates = [1, 1, 2, 2, 3, 3, 4, 4];

const arrayWithoutDuplicates = arrayWithDuplicates.filter(
    (number, index, array) => {
        return index !== array.indexOf(number);
    }
);

console.log(arrayWithDuplicates); // [1, 1, 2, 2, 3, 3, 4, 4]
console.log(arrayWithoutDuplicates); // [1, 2, 3, 4];

What is happening??

filter() keeps or remove elements if the value returned from the callback is true or false
indexOf() tells us where the element was first seen in the array it was called on

The logic is checking if the index of the current element is not the same as the index where it was first seen.

summary of filter()

filter() is used to filter out element in an array.

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