Array.prototype.flat

```js
const arr = [1, [2, 3, [4, 5]], 1, 2, [6, 7]]
Array.prototype.flat = function (deep = 1) {
    let res = []
    deep--
    for (const p of this) {
        if (Array.isArray(p) && deep >= 0) {
            res = res.concat(p.flat(deep))
        } else {
            res.push(p)
        }
    }
    return res
}
console.log(arr.flat(1))
```