Immutable - change elements in array with slice (no splice)

Question

How is possible to change 3/4 elements? Expected output is [1,2,4,3,5]

let list = [1,2,3,4,5];
const removeElement = list.indexOf(3); // remove number 3
list.slice(0, removeElement).concat(list.slice(removeElement+1)) // [1,2,4,5]

...next push number 3 after number 4 without splice

Vesper

But why without splice? Splice is designed for this purpose... – Aedes

@Aedes Array.prototype.splice() is not immutable. It modifies the given array. Looks like OP is looking for an immutable solution. – Employee

@Employee I could swear that the question was not phrased that way when i wrote that comment, could be wrong though. slice().splice() works though, for immutability – Aedes

slice().splice() (copying and then using splice) doesn't work because splice returns the removed elements, not the updated array – Landaulet

just trow the new array created in the last row into some var – Efrainefram

Answer 1

slice doesn't mutate the array on which it operates so you need to assign a value to what it returns

let list = [1,2,3,4,5];
const removeElement = list.indexOf(3); // remove number 3
var newList = list.slice(0, removeElement).concat(list.slice(removeElement+1)) // [1,2,4,5]

If you are prepared to use ES2015 syntax, you can use the spread operator as follows:

const removeElement = list.indexOf(3); // remove number 3
var es6List = [
  ...list.slice(0, removeElement),
  ...list.slice(removeElement+1)
];
console.log(es6List);

fiddle

Plead

Note that this doesn't strictly answer the question as it's missing the final part that pushes 3 after 4. – Seaver

Answer 2

The simplest way to write this is to use the spread operator:

let newList = [...list.slice(0, 2), list[3], list[2], ...list.slice(4)];

Campeche

Edit queue is full but there's an off-by-one error here (should be list[3], list[2] to generate [1,2,4,3,5]) – Commensal

Answer 3

The easer solution might be using filter instead of splice or slice. According to documentation https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter

The filter() method creates a new array with all elements that pass the test implemented by the provided function.

It means the original array stays immutable. The only difference is that in this case, you have to know the value you want to delete instead of index.

let list = [1,2,3,4,5];
list.filter((item) => item !== 3);

Holding

This gets rid of the 3, but you need to inject it back into the array after the 4. That's where you need the splice. – Seaver

Answer 4

var list = [1,2,3,4,5];
var numToRemove = 3;

var removeElementIndex = list.indexOf(numToRemove);
var afterRemoveElement = list[removeElementIndex+1];

list.slice(0, removeElementIndex).concat(afterRemoveElement).concat(numToRemove).concat(list.slice(removeElementIndex+2)) // [1,2,4,3,5]

Beater

Answer 5

Object.assign actually works here

const newList = Object.assign([], list, {
  2: list[3],
  3: list[2],
});

list // [1,2,3,4,5]
newList // [1,2,4,3,5]
newList === list // false

Kinnikinnick

Answer 6

Arrays are objects, use Object.assign() and access elements with property name expressions.

var numToMove = 2;
console.log(Object.assign(list, {[numToMove]: list[numToMove+1]}, 
{[numToMove+1]: list[numToMove]}));
// [1, 2, 4, 3, 5]

Gustafson