// Arbitrarily nested array to flattened array. Mutates.
const flatten = arr => {
    for (let i=0; i < arr.length; Array.isArray(arr[i]) && arr.splice(i, 1, ...arr[i])  || i++){}
    return arr
}

// testing
const nested = [[1,2,[3]], 4]
const flattened = [1,2,3,4]

JSON.stringify(nested) !== JSON.stringify(flattened)
JSON.stringify(flatten(nested)) === JSON.stringify(flattened)
JSON.stringify(flatten(flattened)) === JSON.stringify(flattened)