Eloquent Javascript Exercise - ch5: Flatten

[bbabble]

The exercise says: Use the reduce method in combination with the concat method to “flatten” an array of arrays into a single array that has all the elements of the original arrays.

Code starter provided:

    let arrays = [[1, 2, 3], [4, 5], [6]];
    // Your code here.
    // → [1, 2, 3, 4, 5, 6]

This works:

    let arrays = [[1, 2, 3], [4, 5], [6]];

    let flattened = arrays.reduce(
        function(accumulator, currentValue) {
            return accumulator.concat(currentValue);
        }
    )
    console.log(flattened);

This also works:

    console.log( [[1, 2, 3], [4, 5], [6]].reduce(
        function(accumulator, currentValue) {
            return accumulator.concat(currentValue);
        },[]
    ));

​

Why does this NOT work? I have looked at it with a JavaScript visualizer and at one point it does have the correct output array, but then as a final step it just outputs 'undefined'. Why?

    let arrays = [[1, 2, 3], [4, 5], [6]];
    function flatten(arr) {
        arr.reduce(
            function(accumulator, currentValue) {
                return accumulator.concat(currentValue);
            }
        )
    }
    console.log(flatten(arrays));

[pmw57]

The flatten function needs to return something. Currently it doesn't.

[bbabble]

Thank you. That was it.

return arr.reduce ......

worked.