JSFiddle - React, Tailwind, and code Playground

JavaScript

const input = [1, 2, 3, [4, 5, 6, [8, 9, 10]], 11, [12, 13, 14]];

const flatten = (arr) =>
  arr.reduce((acc, curr) =>
    Array.isArray(curr) ? acc.concat(flatten(curr)) :
    acc.concat(curr), [])

console.log(flatten(input));