JavaScript Array unshift() in an immutable way

Question

How would you implement something like the Array unshift() method so that it creates a new array? Basically something like the Array concat() method but instead of placing the new item at the end, place it in the beginning.

push() adds it to the end. concat() can add to the start or end, depending on how you call it... you can always origArr.slice().doSomethingMutative() also... old.concat(newer) vs newer.concat(old) — dandavis
– dandavis, Commented May 12, 2016 at 23:09
Thank you! I'll check it out.. EDIT: I meant to ask about unshift, sorry about that. — Vlady Veselinov
– Vlady Veselinov, Commented May 12, 2016 at 23:11
array.toSpliced returns a new array without modifying the original array doc eg. const oriArr = [1, 2, 3]; const newItem = 12; const newArr = oriArr.toSpliced(0, 0, newItem); // [12, 1, 2, 3] — tinystone
– tinystone, Commented Aug 15 at 5:03

Alexander O'Mara · Accepted Answer · 2016-05-25 04:48:42Z

54

You can actually accomplish this using the .concat method if you call it on an array of elements you want to put at the front of the new array.

Working Example:

var a = [1, 2, 3];
var b = [0].concat(a);
console.log(a);
console.log(b);

Alternately, in ECMAScript 6 you can use the spread operator.

Working Example (requires modern browser):

var a = [1, 2, 3]
var b = [0, ...a];
console.log(a);
console.log(b);

edited May 25, 2016 at 4:48

answered May 25, 2016 at 4:43

Alexander O'Mara

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