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

insertInterval.js

/**
 * @param {number[][]} intervals
 * @param {number[]} newInterval
 * @return {number[][]}
 */
var insert = function(intervals, newInterval) {
    
    for(let i=0; i<intervals.length; i++){
        
        if(fitsBefore(newInterval, i)){
            return intervals.slice(0, i)
                    .concat([newInterval])
                    .concat(intervals.slice(i));
        }
        
        if(overlaps(newInterval, i)){
            const newStart = Math.min(intervals[i][0], newInterval[0]);
            let j = i;
            while(intervals[j] !== undefined && newInterval[1] >= intervals[j][0]) j++;
            j--;
            const newEnd = Math.max(newInterval[1], intervals[j][1]);
            
            return intervals.slice(0, i)
                    .concat([[newStart, newEnd]])
                    .concat(intervals.slice(j+1));
        }
    }
    
    // if did not insert during loop
    intervals.push(newInterval);
    return intervals;
    
    
    function fitsBefore(tuple, i){
        return tuple[1] < intervals[i][0];
    }
    
    function overlaps(tuple, i){
        return tuple[0] <= intervals[i][1] && tuple[1] >= intervals[i][0];
    }
};