Mutating and Immutable Methods

// non-mutating methods
array.map()
array.slice()
array.concat()
[...array] // spread operator
array.filter()

// mutating methods
array.push()
array.unshift()
array.pop()
array.shift()
array.splice()

Immutable Methods

To keep the integrity of the original array, you need to use .slice() to the original array to make exact value copy of it

const arr = [1, 2, 3];

// immutable
function immutableRemoveElBack(arr) {
  let nArr = arr.slice(0, arr.length-1);
  return nArr;
}

// instantiate deep copy
const newArr = immutableRemoveElBack(arr);

console.log(arr); // [1, 2, 3] untouched
console.log(newArr); // [1, 2] deep copy

Mutating Methods

// original
const arr = [1, 2, 3];

function mutableRemoveElBack(arr) {
  let nArr = arr;
  nArr.pop();
  return nArr;
}

const newArr = mutableRemoveElBack(arr);

console.log(arr); // [1, 2] modified
console.log(newArr); // [1, 2]