Reduce arrays - http://www.careercup.com/question?id=5676298150084608

reduce.js

var input = [[2,6],[3,5],[7,21],[20,21]];
var expectedOutput = [[2,6],[7,21]];

var rangesOverlap = function(range1, range2) {
    return (
        (range1[1] >= range2[0] && range1[1] <=range2[1]) ||
        (range1[0] <= range2[0] && range1[1] >= range2[0])
    );
};

var mergeRanges = function(range1, range2) {
    return [Math.min(range1[0], range2[0]), Math.max(range1[1], range2[1])];
};

var reduced = input.reduce(function(reduced, range) {
    var lastRange = reduced[reduced.length-1];

    if (rangesOverlap(lastRange, range)) {
        reduced.push(mergeRanges(reduced.pop(), range));
    } else {
        reduced.push(range);
    }

    return reduced;
}, [input[0]]);

var reducedString = JSON.stringify(reduced);
var expectedString = JSON.stringify(expectedOutput);

console.log(reducedString, "==", expectedString);
console.log("=>", (reducedString == expectedString));