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Common Array Operations in JavaScript

ยท 3 min read

Array is one of the most common data structures in JavaScript. I will briefly record and summarize some of its operations here.

This paper mainly includes:

  • Create an array
  • Check if it's an array
  • Array-like object and transform it to array
  • Remove duplicate elements

Create an arrayโ€‹

Creating an array is a basic skill, which includes the following methods:

const arr = [1, 2, 3];
const arr2 = new Array(3); // [,,]
const arr3 = new Array(2, 3); // [2, 3]
const arr4 = Array.of(2, 3); // [2, 3]

The one we usually use the most is array literals.

Check if it's an arrayโ€‹

The main way to tell if it's an array is:

// one
// two
arr instanceof Array;
// three === "[object Array]";
// four
arr.constructor === Array;

The first one is the most common way.

Array-like object and transform it to arrayโ€‹

We sometimes encounter arrays that are not real arrays, it is called array-like object. but we can transform it to array. eg: arguments or HTMLCollection.

const eles = document.querySelectorAll("a"); // HTMLCollection
// one;
// two
// three
// four
function toArray(arrayLike) {
let res = [];
for (item of arrayLike) {
return res;
// five
Array.apply(null, eles);
// six
[].concat.apply([], eles);

Remove duplicate elementsโ€‹

Array de-duplication essentially requires that two elements are compared for equality, and if they are, an element is discarded. For accurate judgment, we use for comparison.


The Set object lets you store unique values of any type, whether primitive values or object references.

function unique(arr){
return Array.from(new Set(arr)); // or [ Set(arr)];

reduce with includesโ€‹

  • reduce : executes a reducer function for array element, and returns the final accumulated value.
  • includes : returns true if the value is present in the array.
function unique(arr){
return arr.reduce((acc, cur) => {
if (!acc.includes(cur)) {
return acc;
}, []);

for loop with Mapโ€‹

  • Map : The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value.
function unique(arr) {
const map = new Map();
const ret = [];
for (let i = 0; i < arr.length; i++) {
if (!map.has(arr[i])) {
map.set(arr[i], true);
return ret;

filter with indexOfโ€‹

  • indexOf : returns the first index at which a given element can be found in the array, or -1 if it is not present.
function unique(arr) {
return arr.filter((item, index) => arr.indexOf(item) === index);