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How to manipulate immutably and mutably JavaScript Array with only ES6+

snamiki1212 profile image Nash Updated on ・3 min read

How to manipulate immutably and mutably JavaScript Array with only ES6+

JavaScript is not pure functional programming so some method has a side effect.

When I started to learn JavaScript's methods, every time I confused about which method is immutable/mutable or what is a return value.

Especially, we developers might frequently use Array's methods. So I have always wanted to organize basic Array's methods of how to manipulate mutably and immutably without any library, using pure JavaScript.

Point

Basically, some of JavaScript Array's methods are mutable so a key is a spread operator. We can use a mutable method as if immutably as long as we use a spread operator well.

I believe this is a better way from a perspective of simplicity more than another.

Methods

Here is a summary table.

Action Mutable Immutable
#pop pop(): popped list.slice(-1)
#push push(...arg): lengthNum [...list, ...items]
#shift shift(): shifted [item, ...rest] = list
#unshift unshift( ...arg ): lengthNum [...items, ...list]
#reverse reverse(): reversed [...list].reverse()
#sort sort(): sorted [...list].sort()
#splice / slice splice( startIdx, deleteCount = 1 ) :listFromStartToEnd slice(startIdx, endIdx?)

Check one by one.

pop

Mutable: pop(): item

list = ['a', 'b', 'c']
item = list.pop()
// list: ['a', 'b'], item: 'c'
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Immutable

list = ['a', 'b', 'c']
[item] = list.slice(-1)
// item: 'c'
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push

Mutable: push(...arg): lengthNum

list = ['a', 'b', 'c']
length = list.push('d', 'e')
// list: ['a', 'b', 'c', 'd', 'e'], length: 5
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Immutable

list = ['a', 'b', 'c']
newList = [...list, 'd', 'e']
// newList: ['a', 'b', 'c', 'd', 'e']
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shift

Mutable: shift(): item

list = ['a', 'b', 'c']
item = list.shift()
// list: ['b', 'c'], item: 'a'
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Immutable

list = ['a', 'b', 'c']
[item, ...rest] = list
// item: 'a', rest: ['b', 'c']
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unshift

Mutable: unshift( ...arg ) :lengthNum

list = ['a', 'b', 'c']
length = list.unshift('x')
// list: ['x', 'a', 'b', 'c'], length: 4
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Immutable

list = ['a', 'b', 'c']
newList = ['x', ...list]
// newList: ['x', 'a', 'b', 'c']
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reverse

Mutable: reverse(): reversedList

list = ['a', 'b', 'c']
list.reverse()
// list: ['c', 'b', 'a']
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Immutable

list = ['a', 'b', 'c']
newList = [...list].reverse()
// newList: ['c', 'b', 'a']
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sort

Mutable: sort(): sorted

list = [2, 1, 3]

list.sort((a, b) => a - b) // ASC
// list: [1, 2, 3]

list.sort((a, b) => b - a) // DESC
// list: [3, 2, 1]
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Immutable

list = [2, 1, 3]
asc  = [...list].sort((a, b) => a - b)
desc = [...list].sort((a, b) => b - a)
// asc:  [1, 2, 3]
// desc: [3, 2, 1]
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splice / slice

Mutable: splice( startIdx, deleteCount = 1 ) :listFromStartToEnd

list = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
spliced = list.splice(2, 3)
// [           'c', 'd', 'e'          ] // <= spliced
// ['a', 'b',                'f', 'g' ] // <= list
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Immutable: slice(startIdx, endIdx?)

list = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
sliced = list.slice(2, 5)
remain = [...list.slice(0,2), ...list.slice(5, 7)]
// [           'c', 'd', 'e'          ] // <= sliced
// ['a', 'b',                'f', 'g' ] // <= remain
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Conclusion

Don't hate mutable JavaScript methods, use a spread operator well.

Discussion (4)

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aminmansuri profile image
hidden_dude

I'd call that a "defensive copy" rather than being inmutable.

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snamiki1212 profile image
Nash Author

Thank you comment!

Right! strictly speaking, it's not immutable. I didn't know the word "defensive copy".

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aminmansuri profile image
hidden_dude

I guess it wouldn't be hard in Javascript to create a decorator object that wraps your list / array where you get an error or no-op if you try to mutate it.

That would have the advantage of not needing to copy a potentially huge object.
(though map/reduce/etc.. generally do make copies or at least "stream" the new values)

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aminmansuri profile image
hidden_dude

inmutable would be interesting though.. I have no idea how to do that.