Arrays #3 — Array Methods Usage Reference

JavaScript arrays have 30+ built-in methods. This is a practical reference organized by what they do — not just a list of signatures. Every method includes a runnable example showing why you'd reach for it.

1. Adding & Removing Elements

push(...items) / pop() — End of Array (Stack)

Both mutate the array. push returns the new length; pop returns the removed element.

unshift(...items) / shift() — Beginning of Array (Queue)

O(N) — all elements shift. push/pop are O(1) and preferred for performance.

splice(start, deleteCount?, ...items) — Insert / Remove Anywhere

splice mutates and returns the removed elements.

2. Non-Mutating Access & Slicing

slice(start?, end?) — Extract a Portion

Interview use: Extracting everything except the last item (slice(0, -1) in listFormat), shallow cloning.

join(separator?) — Array → String

Interview use: Turning class name arrays into strings (classnames), building list strings (listFormat).

concat(...values) — Merge Arrays

Does not mutate. Spread [...a, ...b] is the modern alternative.

3. Searching & Finding

indexOf(value, fromIndex?) / lastIndexOf(value) — Position

Uses === comparison. Cannot find NaN (NaN !== NaN).

includes(value, fromIndex?) — Contains Check

Interview use: Checking if a value exists (multi-value filter, class name dedup).

find(callback) / findIndex(callback) — First Match

Returns the first match, not all matches. Use filter for all matches.

filter(callback) / some(callback) / every(callback) — Condition Tests

Interview use: filter in data selection (#30), some in OR filter groups, every for multi-class matching (#24 DOM traversal).

4. Transforming (Non-Mutating)

map(callback) — Transform Each Element

Returns a new array of the same length. Interview use: Deep cloning arrays, transforming data, generating JSX.

reduce(callback, initialValue?) — Accumulate

Interview use: countBy, groupBy, data merging, flatten one-liner.

flat(depth?) / flatMap(callback) — Flatten

Interview use: Interview #12 flatten implementation, transforming and flattening in one pass.

5. Sorting & Ordering

sort(compareFn?) — In-Place Sort

Mutates the original. Clone first with [...arr] if you need immutability.

reverse() — In-Place Reverse

6. Iteration Methods

forEach(callback) — Loop with Side Effects

No return value. Cannot break early. Prefer for...of if you need break/continue.

7. Utility Methods

Array.isArray(value) — Type Check

The only reliable way to check if something is an array. typeof [] returns 'object'.

Array.from(iterable, mapFn?) — Create from Iterable

Array.of(...items) — Create from Arguments

Mutation vs Non-Mutation Summary

Interview Tips

• Clone before mutating — [...arr].sort() not arr.sort() if you need the original.
• includes finds NaN — indexOf doesn't. Know the difference.
• splice is the Swiss Army knife — insert, remove, and replace all in one call.
• reduce can build anything — objects, arrays, maps — it's the most general array method.

Related Articles

• Arrays #2 — flat, flatMap, find, every, some Polyfills
• #2 — Implement map(), filter() & reduce()
• #12 — Implement flatten()