When working with arrays in JavaScript, it’s important to understand the difference between mutating and non-mutating methods.
👉 Mutating methods change the original array.
👉 Non-mutating methods return a new array (or value) without altering the original one.
In this post, we’ll cover non-mutating methods that every developer should know.
📌 Example Array
const numbers = [3, 7, 10, 15, 20, 25]; 1. map() → Transform elements into a new array
const doubledNum = numbers.map((num) => num * 2); console.log(doubledNum); // [6, 14, 20, 30, 40, 50] 2. filter() → Keep only elements that match a condition
const greaterNum = numbers.filter((num) => num >= 10); console.log(greaterNum); // [10, 15, 20, 25] 3. reduce() → Reduce array to a single value
const total = numbers.reduce((acc, cur) => acc + cur, 100); console.log(total); // 180 4. find() → Find the first element that matches
const arrFind = numbers.find((num) => num % 5 === 0); console.log(arrFind); // 10 5. some() → Check if at least one element matches
const arrayCheck = numbers.some((ele) => ele === 30); console.log(arrayCheck); // false 6. every() → Check if all elements match
const positiveNum = numbers.every((num) => num >= 0); console.log(positiveNum); // true 7. includes() → Check if array contains a value
const includesArr = numbers.includes(50); console.log(includesArr); // false 8. indexOf() → Find the index of an element
const indexofArr = numbers.indexOf(15); console.log(indexofArr); // 3 9. flat() → Flatten nested arrays
const flatArr = [1, 2, 3, [50]]; console.log(flatArr.flat()); // [1, 2, 3, 50] 10. flatMap() → Map + flatten in one step
const arrFlatMap = numbers.flatMap((num) => [num * 2]); console.log(arrFlatMap); // [6, 14, 20, 30, 40, 50] 🎯 Final Thoughts
- These methods don’t mutate the original array.
- They’re super useful for functional programming and clean code.
- Once you master these, you’ll find writing JavaScript logic much easier and more predictable.
🔥 Save this list for your next coding session!
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