Source: https://leetcode.com/problems/insert-interval/

You are given an array of non-overlapping intervals intervals where intervals[i] = [starti, endi] represent the start and the end of the ith interval and intervals is sorted in ascending order by starti. You are also given an interval newInterval = [start, end] that represents the start and end of another interval. Insert newInterval into intervals such that intervals is still sorted in ascending order by starti and intervals still does not have any overlapping intervals (merge overlapping intervals if necessary). Return intervals after the insertion.

/**
* @param {number[][]} intervals
* @param {number[]} newInterval
* @return {number[][]}
*/
// O(N) time, O(N) space
var insert = function(intervals, newInterval) {
 let index = 0;
 let result = [];
 
 // Add all the intervals before the new interval aka current interval end < new interval start
 while (index < intervals.length && intervals[index][1] < newInterval[0]) {
   result.push(intervals[index]);
   index++;
 }
 
 // Merge all intervals that intersect with the new interval aka current interval start <= new interval end
 while (index < intervals.length && intervals[index][0] <= newInterval[1]) {
   newInterval[0] = Math.min(intervals[index][0], newInterval[0]);
   newInterval[1] = Math.max(intervals[index][1], newInterval[1]);
   index++;
 }
 result.push(newInterval);
 
 // Add all the intervals after the merged interval 
 while (index < intervals.length) {
   result.push(intervals[index]);
   index++;
 }
 
 return result;
};