Extending string prototype with a function , that returns url relative to gives as an argument.

Url relative to

String.prototype.startsWith = function (input) {
    return this.substring(0, input.length) === input;
};

String.prototype.relativeTo = function (input) {
    var toTop = /..\//gi;
    var abs = /^https?:\/\//i;
    var inCurrent = './';
    var matches;

    if (abs.test(this)) {
        return this.valueOf();
    }

    function getLastSegmentIndex() {
        return (input.lastIndexOf('/') + 1) - (input.length - 1);
    }

    try {
        matches = this.match(toTop).length;
    } catch (e) {
        matches = 0;
    }

    if (!matches) {
        return input.slice(0, -getLastSegmentIndex()) + this;
    } else if (this.startsWith(inCurrent)) {
        return input.slice(0, -getLastSegmentIndex()) + this.replace(inCurrent, '');
    }
    var segments = input.split('/');
    var i = 0;
    for (; i < matches + 1; i++) {
        segments.pop();
    }
    segments.push((this.replace(toTop, '')));
    return segments.join('/');
};

console.log('case 1 ', '../d.html'.relativeTo('/a/b/c.html')); // output case 1 /a/d.html
console.log('case 2 ', 'g.html'.relativeTo('/a/b/c.html')); // output case 2 /a/b/g.html
console.log('case 3 ', './f/j.html'.relativeTo('/a/b/c.html')); // output case 3 /a/b/f/j.html
console.log('case 4 ', '../../k/w.html'.relativeTo('/a/b/c.html')); // output case 4 /k/w.html
console.log('case 5 ', 'https://www.google.com'.relativeTo('/a/b/c.html')); // output case 5 https://www.google.com