Filtering Arrays in JavaScript

To find the scripts in the data set that are still in use, the following function might be helpful. It filters out the elements in an array that don’t pass a test.

function filter(array, test) {

let passed = [];

for (let element of array) {

if (test(element)) {




return passed;


console.log(filter(SCRIPTS, script =>;

// → [{name: “Adlam”, …}, …]

The function uses the argument named test, a function value, to fill a “gap” in the computation—the process of deciding which elements to collect.

Note how the filter function, rather than deleting elements from the existing array, builds up a new array with only the elements that pass the test. This function is pure. It does not modify the array it is given.

Like forEach, filter is a standard array method. The example defined the function only to show what it does internally. From now on, we’ll use it like this instead:

console.log(SCRIPTS.filter(s => s.direction == “ttb”));

// → [{name: “Mongolian”, …}, …]

Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming,

No Starch Press; 3rd edition.

Leave a Reply

Your email address will not be published. Required fields are marked *