selfFlat.js

const selfFlat = function (depth = 1) {
    let arr = Array.prototype.slice.call(this)
    if (depth === 0) return arr
    return arr.reduce((pre, cur) => {
        if (Array.isArray(cur)) {
            return [...pre, ...selfFlat.call(cur, depth - 1)]
        } else {
            return [...pre, cur]
        }
    }, [])
}

Array.prototype.selfFlat = selfFlat;