Flatten Nested Array with ES2015

flatten.js

// This example uses ES2015 features. Use Babel to compile down to JS, or see the example at http://codepen.io/sardell/pen/VjGYRz

// example array provided
var example = [[1,2,[3]],4];

function flatten_array(nested) {
  // concatenate using the spread operator
  const flat = [].concat(...nested)
  // repeat flatten_array function until nested array is flat
  return flat.some(Array.isArray) ? flatten_array(flat) : flat;
}

console.log(flatten_array(example));