What is array filter in JavaScript

What is the filter method on a array

The filter() method takes in a function expression as a argument. It has to return a value that evaluate as true or false so the filter() method knows to keep the element or remove it from the array. filter() creates a new array and returns the new array, it does not change the original array it is called on.

let numbers = [3, 10, 40, 3]
let newNumbers = arr.filter(function (number) {
return number >= 10;
console.log(numbers) // [3, 10, 40, 3]
conosle.log(newNumbers) // [10, 40]

When the filter() method gets invoked its callback gets 3 arguments

  • the value element
  • the index of the element
  • the original array being looped through

practical uses of filter in JavaScript

makes it really easy to remove duplicates in a array.

// array with duplicates we want to remove them.
let arrayWithDuplicates = [1, 1, 2, 2, 3, 3, 4, 4];

// we have access to the current element of the array,
// the current index of the array, and the original array that is being looped through
let arrayWithoutDuplicates = arrayWithDuplicates.filter(function (element, index, arr) {
return index !== arr.indexOf(element);
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.

keep elements that have a specific class

Let say you are making a todo app and want to show todos that are finish. We also decided that to tell if it is finish or not by seeing if it has the class of done.

// returns a array of elements with a class of todo
var arr = todosToArray();
// remove all elements that does not have the className of done
var removeCheckedElements = arr.filter(function (element) {
return element.className.includes('done');

filter() is removng all elements in the array that have done as a class. includes() just checks if whatever is passed in as a argument is included in a string.

The power of filter

filter() is powerful when you are trying to filter a array to only have certain items.

the basics

A callback is needed

  • checks every element in the array and checks that to a condition to know if it should be kept or removed from the array.
  • have access to the element, the index, and the original array being looped.
  • makes and returns a new array, doesn’t alter the original array.

happy coding :)

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