[javascript] Merge/flatten an array of arrays

Update: it turned out that this solution doesn't work with large arrays. It you're looking for a better, faster solution, check out this answer.


function flatten(arr) {
  return [].concat(...arr)
}

Is simply expands arr and passes it as arguments to concat(), which merges all the arrays into one. It's equivalent to [].concat.apply([], arr).

You can also try this for deep flattening:

function deepFlatten(arr) {
  return flatten(           // return shalowly flattened array
    arr.map(x=>             // with each x in array
      Array.isArray(x)      // is x an array?
        ? deepFlatten(x)    // if yes, return deeply flattened x
        : x                 // if no, return just x
    )
  )
}

See demo on JSBin.

References for ECMAScript 6 elements used in this answer:


Side note: methods like find() and arrow functions are not supported by all browsers, but it doesn't mean that you can't use these features right now. Just use Babel — it transforms ES6 code into ES5.