Merge Intervals in JavaScript
The code is mostly readable and clear:


the variable names are descriptive (for the most part - x is a little unclear)
there is good use of const and let instead of var


Some of the lines are a little lengthy - the longest line appears to be 117 characters long (excluding indentation):


newIntervals[newIntervals.length - 1][1] = Math.max(newIntervals[newIntervals.length - 1][1], sortedIntervals[i][1]);



I considered suggesting that a for...of loop be used to replace the for loop in the imperative solution after seeing the suggestion to use arr.entries() in this answer which would allow the use of a variable like interval instead of sortedIntervals[i], though when comparing in FF and chrome with this jsPerf test it seems that would be slower and thus less optimal, perhaps because each iteration would have an added function call.

When intervals.sort() is called the array is sorted in-place so intervals could be used instead of sortedIntervals, however it does seem that using sortedIntervals. That would reduce the storage by \$O(n)\$.