Adding arrays in JavaScript usually means combining two or more arrays into one result. The most common tools for that job are concat(), spread syntax, and Set when you also want to remove duplicates.
This is a practical pattern for list merging, report building, and data cleanup. If you are going to dedupe values afterward, deduplicating a JavaScript array is the next related topic.
Tested on: Node.js v20.18.2. A short note after each runnable snippet describes what you should see in the console.
Method 1: Add arrays with concat()
concat() joins arrays and returns a new array without changing the originals.
const array1 = [1, 2, 3];
const array2 = [4, 5, 6];
const combined = array1.concat(array2);
console.log("add-arrays:", combined.join(","));You should see one line logging add-arrays: 1,2,3,4,5,6.
Use this when you want to merge arrays and keep the source arrays untouched.
Method 2: Add arrays with spread syntax
Spread syntax gives you a short way to build a new array from existing arrays.
const array1 = [1, 2, 3];
const array2 = [4, 5, 6];
const combined = [...array1, ...array2];
console.log("add-arrays-spread:", combined.join(","));You should see one line logging add-arrays-spread: 1,2,3,4,5,6.
This is the cleanest approach when you want a direct literal-style merge.
Method 3: Add arrays and remove duplicates
Set removes duplicate primitive values after the arrays are combined.
const array1 = [1, 2, 3];
const array2 = [3, 4];
const unique = [...new Set(array1.concat(array2))];
console.log("add-arrays-set:", unique.join(","));You should see one line logging add-arrays-set: 1,2,3,4.
This is the right pattern when you want a merged array without repeated primitive values.
Summary
To add arrays in JavaScript, use concat() when you want a familiar merge, use spread syntax when you want compact code, and use Set when you also need to remove duplicates. Together these patterns cover most merge-and-dedupe tasks without extra libraries.
