concat() and join()

Syntax

arr.concat(otherArr)
arr.join(separator)

Examples

const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];

const merged = arr1.concat(arr2);
console.log(merged); // [1, 2, 3, 4, 5, 6]
console.log(arr1);   // [1, 2, 3] - unchanged

// concat() can also merge multiple arrays and add individual values
const combined = arr1.concat(arr2, [7, 8], 9);
console.log(combined); // [1, 2, 3, 4, 5, 6, 7, 8, 9]

const words = ["Hello", "World", "!"];

console.log(words.join(" ")); // "Hello World !"
console.log(words.join());    // "Hello,World,!" - default separator is a comma
console.log(words.join(""));  // "HelloWorld!" - no separator

const path = ["usr", "local", "bin"];
console.log(path.join("/")); // "usr/local/bin"

Best practices

• Prefer the spread operator ([...arr1, ...arr2]) over concat() in modern code - it reads more clearly, though both achieve the same result
• Use join() instead of manually looping and building a string when converting an array to display text
• Choose a join() separator that matches your output format - commas for CSV-like data, spaces for sentences, "/" for paths
• Remember join() converts non-string elements using their default string representation, which may need formatting first for numbers or objects

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation