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Javascript cheat sheet: .slice() and .splice()

#codenewbie #javascript #beginners #tutorial

I recently came across a twitter post where the user challenged others if they knew the difference between the js methods .slice() and .splice() without googling them. A lot of people mixed them up.

So I've created this simplified cheat sheet of everything you need to know about the 2 methods.

What you should know

  • .slice() and splice() are both array methods.

  • both of them return arrays as outputs

  • the key is to pay attention to what they return as the output array and if they mutate the original array (they one on which the were applied to)

    1) .slice()

  • .slice() returns selected elements in an array, as a new array. Selection is done by index.

  • .slice() does not affect the original array

With splice() you can select one or more elements.

For example: This code snippet returns 3 (which is at index 2)

[1,2,3,4,5].slice(2) // returns 3 (which is at index 2)
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and this code snippet returns 2,3,4 (from index 1 to 4 exclusively)

[1,2,3,4,5].slice(1,4) // returns 2,3,4 (from index 1 to 4 exclusively)
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2) .splice()

  • .splice modifies the original array (adds and/or removes elements from the array)

  • The method returns the original array **modified**

With splice you can:

  • remove elements 
[1,2,3,4,5].splice(3) // returns [1,2,3] (first 3 elements)
[1,2,3,4,5].splice(2,2) // at position 2 remove 2 elements (returns [1,2,5])
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  • add elements 

[1,2,5].splice(2,0,3,4) // at postion 2 add 2 elements (returns [1,2,3,4,5])
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  • remove and add elements at the same time 
// at position 2, remove 1 elements and add 2 new items
[1,2,3,4,5].splice(2,2,"HTML","CSS") // returns [1,2,"HTML","CSS",5]
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Remember: .splice() mutates the array, .slice() does not!

Hope this is helpful to someone 💜

Oldest comments (4)

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orliesaurus profile image
orliesaurus •

Helpful, thank you!

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hirenvadher954 profile image
Hiren Vadher • • Edited

Sometimes I think Why JS has so many confusing terms. BTW Great info thanks for sharing.

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jackmellis profile image
Jack •

Slice is underrated as a way to safely do normally-mutative calls.
Get last item:

const last = arr.slice().pop()
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First item:

const first = arr.slice.shift()
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Reverse a list without mutating the original:

const reversed = arr slice().reverse()
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Also

The method returns the original array modified

This isn't quite right. The splice function actually returns an array of elements that were deleted from the original array

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jackmellis profile image
Jack •

Also also your slice(2) example would return [3,4,5]. With one argument, slice returns all elements starting from that index

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