This title could have been "how not to get confused between JavaScript's splice and slice," because I can never remember the difference between the two.

So I am hoping this trick will help both me and you in the future:

S (p) lice = Slice + (p) => Slice + in (p) lace

Array.prototype.slice()

Array.prototype.slice() is used to slice an array from the start point to the end point, excluding the end.

As the name suggests, it is used to slice elements out of an array. But unlike slicing a cake, slicing an array does not cut the actual array, but keeps it unmodified (infinite cake!).

arr.slice(start, [end])

Rules

  1. A new array is returned and the original array is unmodified.
  2. If end is omitted, end becomes the end (last element) of the array.
  3. If start is -ve, the elements are counted from the end.
const infiniteCake = ['🍰','🍰','🍰','🍰','🍰','🍰']

let myPieceOfCake = infiniteCake.slice(0,1) // ['🍰']
let yourDoublePieceOfCake = infiniteCake.slice(0,2) // (2) ["🍰", "🍰"]
console.log(infiniteCake) //['🍰','🍰','🍰','🍰','🍰','🍰']

As you see, inifinteCake is unmodified.

Array.prototype.splice()

Splice does operations in place, which means it modifies the exisiting array.

In addition to removing elements, splice is also used to add elements. Splice is the real world cake "slice":

arr.splice(start, [deleteCount, itemToInsert1, itemToInsert2, ...])

Rules

  1. Operations are performed in place.
  2. An array is returned with the deleted items.
  3. If start is -ve, the elements are counted from the end.
  4. If deleteCountis omitted,the elements until the end of the array are removed.
  5. If items to insert such as itemToInsert1 are omitted, elements are only removed.
const cake = ['🍰','🍰','🍰','🍰','🍰','🍰'];
let myPieceOfCake = cake.splice(0, 1) // ["🍰"]
console.log(cake) // (5) ["🍰", "🍰", "🍰", "🍰", "🍰"]

let yourDoublePieceOfCake = cake.splice(0,2) //(2) ["🍰", "🍰"]
console.log(cake) //(3) ["🍰", "🍰", "🍰"]

Here, cake is modified and reduces in size.

Code Examples

const myArray  = [1,2,3,4,5,6,7] 

console.log(myArray.slice(0))       // [ 1, 2, 3, 4, 5, 6, 7 ]
console.log(myArray.slice(0, 1))    // [ 1 ]
console.log(myArray.slice(1))       // [ 2, 3, 4, 5, 6, 7 ]
console.log(myArray.slice(5))       // [ 6, 7 ]
console.log(myArray.slice(-1))      // [ 7 ]
console.log(myArray)                // [ 1, 2, 3, 4, 5, 6, 7 ]



const secondArray = [10, 20, 30, 40, 50]

console.log(secondArray.splice(0, 1))   // [ 10 ] : deletes 1 element starting at index 0
console.log(secondArray.splice(-2, 1))  // [ 40 ] : deletes 1 element starting at index end-2 
console.log(secondArray)                // [ 20, 30, 50 ]
console.log(secondArray.splice(0))      // [ 20, 30, 50 ] : deletes all elements starting at index 0
console.log(secondArray)                // []
console.log(secondArray.splice(2, 0, 60, 70)) // [] : deletes 0 elements starting at index 2 (doesn't exist so defaults to 0) and then inserts 60, 70
console.log(secondArray)                // [60, 70]

TL;DR

Use splice if the original array needs to be modified, or elements need to be added.

Use slice for removing elements if the original array should not be modified.


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