Pure function that does not generate side effect

pure_function_no_side_effect.js

// In this example i want to clear out the difference between function with side effect and a function without side effect
let arr = [1,2,3];
function removeLastItem(input) {
  input.pop();
}

removeLastItem(arr); 
console.log(arr); // output [1,2] (this function changes the orignal variable from [1,2,3] -> [1,2])
// above execution has side effect as it mutate arr which belong to the outside world.

function immutablyRemoveLastItem(input) {
  const newArray = [].concat(input); // concat method copies the value to new variable.
  return newArray.pop();
}

let newArr = [1,2,3];

console.log(immutablyRemoveLastItem(newArr)); // output [1,2]
console.log(newArr); // output [1,2,3] -> the above function does not have side effect as it not modify the orignal input instead it copies
// and alter the copied array.