Most efficient way to prepend a value to an array

javascript

prompt-engineering

performance

best-practices

byAnton Shumikhin·Feb 28, 2025

For quick and dirty, use the unshift method:

let array = [2, 3, 4];
array.unshift(1); // Now array is [1, 2, 3, 4], imagine that!

If you're all about not messing with the original, unleash the power of the spread operator ...:

let array = [2, 3, 4];
array = [1, ...array]; // New array: [1, 2, 3, 4]. The old one? Untouched. Mic drop.

Heavy-duty prepending

unshift and ... are great and all, but they might break a sweat with large data or loop-a-loops.

Array#unshift is cool, but it quirks when you throw large arrays at it.
... — looks neat, behaves well, until it finds itself in a loop or a big data party.

Now, fear not, for you have options:

Linked Lists: These guys are not afraid of some heavy prepend hacking.

Batch prepend: Got a bunch of guys to add to the front? Round 'em up in an array and feed it to unshift.apply:

let extraHead = [0, 1];
Array.prototype.unshift.apply(array, extraHead);
// Hope you didn't expect to see 101 Dalmatians here.

Immutability: Don't want to mess with your original array? Be smart:

const prepend = (value, array) => [value].concat(array);
const newArray = prepend(1, originalArray); // newArray: [1, old_stuff]. As clean as an elf's ears.

Prepends and memory diet

Prepending values = shuffling things around. The space cost? At least O(N). That's how the memory cookie crumbles.

Benchmark it like it's hot

Your use case is unique. Don't trust me, don't trust benchmarks on the web. Test it out and see who wins your use case race.

Beyond arrays: the final frontier

When regular arrays just don't cut it, let's get fancy:

Concatenation: It's a create-then-destroy approach, works with immutable structures but might be a slowpoke for large arrays.
Queues: Prepending often? Consider an upgrade to a queue, it's like a VIP list for both ends.
Chunks: Break up your data into smaller arrays. Manageable? Check. Readable? You bet.
Memory layouts: Understanding your JavaScript engine can take you places, literally. Some engines give certain array patterns the red carpet treatment.