CODE EXAMPLE FOR JAVASCRIPT

JavaScript flatten an array

let arr = [
    [1, 2],
    [3, 4],
    [5, 6][7, 8, 9],
    [10, 11, 12, 13, 14, 15]
];
let flatArray = arr.reduce((acc, curVal) => {
    return acc.concat(curVal)
}, []);

//Output:  [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 ]