I needed this function this morning so I thought I’d share it in case someone else does too.

function RemoveBaseUrl(url) {
    /*
     * Replace base URL in given string, if it exists, and return the result.
     *
     * e.g. "http://localhost:8000/api/v1/blah/" becomes "/api/v1/blah/"
     *      "/api/v1/blah/" stays "/api/v1/blah/"
     */
    var baseUrlPattern = /^https?:\/\/[a-z\:0-9.]+/;
    var result = "";
    
    var match = baseUrlPattern.exec(url);
    if (match != null) {
        result = match[0];
    }
    
    if (result.length > 0) {
        url = url.replace(result, "");
    }
    
    return url;
}