let arr = [1, [2, [3, [4, 5]]], 6];
let str = JSON.stringify(ary);

arr.flat(Infinity);

注意：flat和flatMap方法为ES2019(ES10)方法，目前还未在所有浏览器完全兼容。

str.replace(/(\[\]))/g, '').split(',');

str = str.replace(/(\[\]))/g, '');
str = '[' + str + ']';
var ary = JSON.parse(str);

let result = [];
let fn = function(ary) {
  for(let i = 0; i < ary.length; i++) }{
    let item = ary[i];
    if (Array.isArray(ary[i])){
      fn(item);
    } else {
      result.push(item);
    }
  }
}

function flatten(ary) {
  return ary.reduce((pre, cur) => {
    return pre.concat(Array.isArray(cur) ? flatten(cur) : cur);
  })
}
let ary = [1, 2, [3, 4], [5, [6, 7]]]
console.log(ary.MyFlat(Infinity))

while (ary.some(Array.isArray)) {
  ary = [].concat(...ary);
}