test for Theorem

flatten.js

/*

 in order to run the tests install
 https://github.com/davidchambers/doctest either globally or locally
 and run `$ doctest theorem.js`

 */

// > flatten([3])
// [3]
// > flatten([1, 2, 3])
// [1, 2, 3]
// > flatten([[[2], 3]])
// [2, 3]
// > flatten([])
// []
// > flatten(NaN)
// NaN
// > flatten([[1, 2, [3]], 4])
// [1, 2, 3, 4]
function flatten(nested) {
    // propagate unhandled values
    if (!Array.isArray(nested)) {
        return nested;
    }
    // here we have an array
    function callback(accum, current) {
        if (Array.isArray(current)) {
            return accum.concat(flatten(current));
        } else {
            return accum.concat([current]);
        }
    }
    return nested.reduce(callback, []);
}