JSFiddle - React, Tailwind, and code Playground

JavaScript

const arr = [1, [2, [3, [[4]]]], [5.45, [215.643]], [[6]]];

function flatten(input) {
  var result = [];

  input.forEach((value) => {
    if ( Array.isArray(value) )
      result.push(...flatten(value));
    else
      result.push(value);
  });

  return result;
};

console.log(flatten(arr));