// for example i use two arrays with ranges, but in reality they are n (>= 1)

var numbersRanges1 = [

    {start: 100, end: 120},

    {start: 180, end: 200},

    {start: 400, end: 500}

];

​

var numbersRanges2 = [

    {start: 10, end: 80},

    {start: 150, end: 220},

    {start: 480, end: 500}

];

​

// result should look like

var expected = [

    {start: 0, end: 10},

    {start: 80, end: 100},

    {start: 120, end: 150},

    {start: 220, end: 400},

    {start: 500, end: 600}

];

​

var boundary = {

    start: 0,

    end: 600

};

​

// merge arrays

var mergedRanges = numbersRanges1.concat(numbersRanges2);

​

​

// sort by start

function sortByStart(a, b){

    return a.start - b.start;

}

​

mergedRanges = mergedRanges.sort(sortByStart);

​

// group overlapping ranges

for(var i = 1; i < mergedRanges.length; i++){

    var range1 = mergedRanges[i-1];

    var range2 = mergedRanges[i];

​

    if((range1.start <= range2.end) && (range1.end >= range2.start)){

        range2.start = Math.min(range1.start, range2.start);

        range2.end = Math.max(range1.end, range2.end);

        mergedRanges.splice(i-1, 1);

    }

}

​

// go throw merged ranges and save ranges between in addition array

var freeRanges = [];

​

if(mergedRanges[0].start > boundary.start){

    freeRanges.push({

        start: boundary.start,

        end: mergedRanges[0].start

    });

}

​

for(var i = 1, mergedLen = mergedRanges.length; i <  mergedLen; i++){

    freeRanges.push({

        start: mergedRanges[i-1].end,

        end: mergedRanges[i].start

    });

}

​

if(mergedRanges[mergedLen-1].end < boundary.end){

    freeRanges.push({

        start: mergedRanges[mergedLen-1].end,

        end: boundary.end

    });

}

​

console.log(freeRanges);

console.log(expected);