javascript append to array

To append items to an array in JavaScript, the most common and efficient methods are:

`push()` Method

Adds one or more elements to the end of an array and returns the new length. It modifies the original array.

let arr = ['first', 'second'];
arr.push('third'); // ['first', 'second', 'third']
arr.push('fourth', 'fifth'); // ['first', 'second', 'third', 'fourth', 'fifth']

To append another array, use the spread operator (...) with push():

let arr1 = ['first', 'second'];
let arr2 = ['third', 'fourth'];
arr1.push(...arr2); // ['first', 'second', 'third', 'fourth']

⚠️ Important: Do not assign the return value of push() back to the array (e.g., arr = arr.push(...)), as it will overwrite the array with the new length (a number).

Spread Operator (`...`)

Creates a new array without modifying the original, enabling immutable updates.

let arr = ['first', 'second'];
let newArr = [...arr, 'third']; // ['first', 'second', 'third']

This is ideal when you want to preserve the original array.

`concat()` Method

Combines arrays and returns a new array without modifying the originals.

let arr1 = ['first', 'second'];
let arr2 = ['third', 'fourth'];
let combined = arr1.concat(arr2); // ['first', 'second', 'third', 'fourth']

`unshift()` Method

Adds elements to the beginning of an array (modifies original).

let arr = ['second', 'third'];
arr.unshift('first'); // ['first', 'second', 'third']

`splice()` Method

Inserts elements at a specific index (useful for inserting in the middle).

let arr = ['first', 'third'];
arr.splice(1, 0, 'second'); // ['first', 'second', 'third']

Summary:
Use push(...) for appending to the end (modifies original).
Use [...arr, item] for immutable appending.
Use concat() for merging arrays without mutation.
Use splice() for inserting at specific positions.

How to append something to an array?

stackoverflow.com › questions › 351409 › how-to-append-something-to-an-array

Use the Array.prototype.push method to append values to the end of an array:

// initialize array
var arr = [
  "Hi",
  "Hello",
  "Bonjour"
];

// append new value to the array
arr.push("Hola");

console.log(arr);

You can use the push() function to append more than one value to an array in a single call:

// initialize array
var arr = ["Hi", "Hello", "Bonjour", "Hola"];

// append multiple values to the array
arr.push("Salut", "Hey");

// display all values
for (var i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

Note that the push() method returns the updated length of the array.

Update

If you want to add the items of one array to another array, you can use firstArray.concat(secondArray):

var arr = [
  "apple",
  "banana",
  "cherry"
];

// Do not forget to assign the result as, unlike push, concat does not change the existing array
arr = arr.concat([
  "dragonfruit",
  "elderberry",
  "fig"
]);

console.log(arr);

Update

Just an addition to this answer if you want to prepend any value to the start of an array (i.e. first index) then you can use Array.prototype.unshift for this purpose.

var arr = [1, 2, 3];
arr.unshift(0);
console.log(arr);

It also supports appending multiple values at once just like push.

Update

Another way with ES6 syntax is to return a new array with the spread syntax. This leaves the original array unchanged, but returns a new array with new items appended or prepended, compliant with the spirit of functional programming.

const arr1 = [
  "Hi",
  "Hello",
  "Bonjour",
];
const arr2 = [
  "Ciao",
  "Hej",
  "Merhaba",
];

const newArr1 = [
  ...arr1,
  "Salut",
];
const newArr2 = [
  "Salut",
  ...arr2,
];
const newArr3 = [
  ...arr1,
  ...arr2,
];

console.log(newArr1, newArr2, newArr3);

freeCodeCamp

forum.freecodecamp.org › t › append-array-to-array › 45740

Append array to array - The freeCodeCamp Forum

October 15, 2016 - Can't seem to find the answer to this, but I'm trying to use push to add an array on the end of another, not insert it. So myArray.push(otherArray); gives me something like ["a","b",["c","d"]] but what I'm tryin…

freeCodeCamp

freecodecamp.org › news › javascript-append-to-array-a-js-guide-to-the-push-method-2

JavaScript Append to Array: a JS Guide to the Push Method

April 19, 2021 - Sometimes you need to append one or more new values at the end of an array. In this situation the push() method is what you need. The push() method will add one or more arguments at the end of an array in JavaScript: let arr = [0, 1, 2, 3];

Discussions

javascript - How to append something to an array? - Stack Overflow

How do I append an object (such as a string or number) to an array in JavaScript? More on stackoverflow.com

stackoverflow.com

Three ways to append an item to an array (Mutative)

myArray = [...myArray, 'Pig'] :) More on reddit.com

r/learnjavascript

131

September 1, 2022

How do I append to an array inside a json file in node?

If it's stored as JSON, you'd read the JSON in, parse it to JavaScript structures using JSON.parse, push the new messages to the JSON, reserialize it back to JSON, then re-write to to disk. This is quite inefficient though. It would be better to use a database. More on reddit.com

r/learnjavascript

December 26, 2022

Check checkbox check state and append to array

Since you are using jQuery, it would be easiest to leverage its :checked selector , e.g.: $(".form-check input[type=checkbox]:checked") would return a jQuery collection containing all checked checkboxes inside .form-check element(s). Once you have that collection, you can use rest of jQuery methods, or convert it to Array and use plain JS or other library's methods, something like this example (note that you can inline getChecked function right into .click(), but I thought it was worth extracting it). More on reddit.com

r/learnjavascript

January 23, 2018

Videos