Flatten array in JavaScript

flatten.js

// Input: 
//   [[1,2,[3]],4]
// Output:
//   [1,2,3,4]
Array.prototype.flatten = function() {
  return this.reduce(function(previousValue, currentValue) {
    let value = Array.isArray(currentValue) ? currentValue.flatten() : currentValue;
    return previousValue.concat(value);
  }, new Array());
}