js array recursively use

Fibonacci sequence:

1,1,2,3,5,8,13,21,34...

function fi(n){if(n == 1 || n == 2) return 1;return fi(n-1) + fi(n-2)}console.log(fi(3)) // 2

Array sum:

function sum(arr){if(arr.length == 0){return 0}else if(arr.length == 1){return arr[0]}else{return arr[0] + sum(arr.slice(1))// slice(1) intercepts the data from the first to the end of the array}}console.log(sum([1,2,3])) // 6

Array flattening:

const res = [];const fn = arr => {for (let i = 0; i < arr.length; i++) {if (Array.isArray(arr[i])) {fn(arr[i]);} else {res.push(arr[i]);}}}fn([1,[2,3]]);