Flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4]

flattenArray.js

// Write some code, that will flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].

function flattenArray(arr) {
    if (!Array.isArray(arr)) {
        return 'Please enter an array'
    }
    return arr.reduce((acc, val) => Array.isArray(val) ? acc.concat(flattenArray(val)) : acc.concat(val), [])
}

// console.log(flattenArray(4)) // will return 'Please enter an array'
console.log(flattenArray([[1, 2, [3]], 4]))