flattenArray

flatten_array.js

// recursive function to flatten an n-level deep array
function flattenArray(array, result = []) {
    var arrayLength = array.length;

    for (var i = 0; i < arrayLength; i++){
        if (array[i] instanceof Array) {
            // if item is an Array, call the function recursevely
            result.concat(flattenArray(array[i], result));
        } else {
            // if item is not an Array, simply add it to final result
            result.push(array[i]);
        }
    }

    return result;
}

// support function to test the results
function test(input, expectedOutput) {
    var result = flattenArray(input);

    if (result.length !== expectedOutput.length) {
        throw "Expected: " + expectedOutput + ". Actual result: " + result;
    }

    for (var i = 0, l = result.length; i < l; i++) {
        if (result[i] !== expectedOutput[i]) {
            throw "Expected: " + expectedOutput + ". Actual result: " + result;
        }
    }

    return true;
}

test([], []);
test(
    [1, [2], 3],
    [1, 2, 3]
);
test(
    [[1, 2], 3],
    [1, 2, 3]
);
test(
    [[[1], 2], 3],
    [1, 2, 3]
);
test(
    [[[[1]], 2], 3],
    [1, 2, 3]
);
test(
    [[1], [2], [3]],
    [1, 2, 3]
);
test(
    [[[[1], [2]], [3]]],
    [1, 2, 3]
);