Simple library to check if intervals intersect. Support open, closed and partially closed intervals. Dedicated to Alex. Tests are here https://gist.github.com/benzel/ffb0851c6e22469cf662

intervals.js

Boundary = function (value, type) {
    this.value = value;
    this.type = type;
}

Boundary.weightTable = {
    ")": 1,
    "]": 2,
    "[": 2,
    "(": 3
}

Boundary.prototype.lessThan = function (boundary) {
    if (this.value < boundary.value) {
        return 1;
    } else if (this.value > boundary.value) {
        return -1;
    } else {  // this.value === interval.value
        var w1 = Boundary.weightTable[this.type];
        var w2 = Boundary.weightTable[boundary.type];
        if (w1 === w2) {
            return 0;
        } else if (w1 < w2) {
            return 1;
        } else {
            return -1;
        }
    }
}

Interval = function (left, right) {
    if (left.lessThan(right) < 0) {
        throw left + " > " + right;
    }
    this.left = left;
    this.right = right;
}

Interval.prototype.intersects = function (interval) {
    // Sort
    var first;
    var second;
    if (this.left.lessThan(interval.left) >= 0) {
        first = this;
        second = interval;
    } else {
        first = interval;
        second = this;
    }
    if (first.right.lessThan(second.left) > 0) {
       return false;
    } else {
       return true;
    } 
}

Interval.makeOpen = function (left, right) {
    return new Interval(new Boundary(left, "("),
                        new Boundary(right, ")"));
}

Interval.makeClosed = function (left, right) {
    return new Interval(new Boundary(left, "["),
                        new Boundary(right, "]"));
}

Interval.makeOpenLeft = function (left, right) {
    return new Interval(new Boundary(left, "("),
                        new Boundary(right, "]"));
}

Interval.makeOpenRight = function (left, right) {
    return new Interval(new Boundary(left, "["),
                        new Boundary(right, ")"));
}