Flatten the deep nested array

flatten.js

/**
 * Function to flatten the deep array
 */

var sampleArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]];

 // Solution for single level nested array
[].concat.apply([], sampleArray);

// If we want deep flatten then we can use recursive function strategy
function flatten(inputArray) {
  return inputArray.reduce(function (a, b) {
    return a.concat(Array.isArray(toFlatten) ? flatten(b) : b);
  }, []);
}


// Tests
var sampleArray2 = [[1, 2],[3, 4, 5], [6, 7, [8, [9]]]];
console.log(flatten(sampleArray2));
// [1, 2, 3, 4, 5, 6, 7, 8, 9]