Which JavaScript Array functions are mutating?

Question

I am writing an Array-derived class in JavaScript and need to know which functions to overload so that I can be aware of changes made to the array.

I know Array.push() and Array.splice() are mutating. Is there a definitive list of any others?

Does this answer your question? How to remember if a method mutates the original array? — Eyeslandic
– Eyeslandic, Commented Jul 18, 2022 at 13:01

Jacob · Accepted Answer · 2016-12-21 22:26:15Z

97

You can find the list on MDN as Mutator methods (along with Accessor and Iteration methods):

edited Dec 21, 2016 at 22:26

Jacob

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answered Jan 25, 2012 at 20:40

Jonathan Lonowski

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3 Comments

zamnuts Over a year ago

Mutator link is currently incorrect, it is now: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

Jonathan Lonowski Over a year ago

@zamnuts Thanks. And, looks like all the links needed some updating.

zamnuts Over a year ago

looks like the mutator method list was removed from MDN docs. see this SO answer instead: stackoverflow.com/a/54836218/1481489

Sooraj · Accepted Answer · 2019-01-23 12:17:45Z

11

I found this website called Doesitmutate

Have the list of all functions - and tells whether it mutates or not.

answered Jan 23, 2019 at 12:17

Sooraj

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Comments

Vinnie James · Accepted Answer · 2017-04-11 22:02:05Z

10

You can also use .concat(), before using your mutation method, to ensure you are not mutating your arrays, eg

const dontMutateMe = [4,5,1,2,3];
const sortArray = dontMutateMe.concat().sort(...)

answered Apr 11, 2017 at 22:02

Vinnie James

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4 Comments

Freewalker Over a year ago

With ES2015 it's just: const sortArray = [ ...dontMutateMe ].sort(...);

jasonleonhard Over a year ago

As nice as ES6 is, ES6 is not always available, luckily concat is. Good to know concat works well in those cases. Just remember, it is not always best to create a whole new array to do basic operations, there is a cost to copying.

rickysullivan Over a year ago

Correct me if I'm wrong @jasonleonhard, but woudln't const sortArray = dontMutateMe.concat() create a new array just as const sortArray = [ ...dontMutateMe ] would?

Vinnie James Over a year ago

Yes, thats what @jasonleonhard said, concat and spread both clone your array

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