Get Web Url by GUID sharepoint client object model

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SharePoint2010

Hi experter,
i'm newbie in Sharepoint and have Problem by getting lists and subsites using siteid
with javascript:
so topsite->subsite1,subsite2
subsite1->subsite11,subsite12,subsite13
subsite2->subsite21,subsite22...
I have guid at subsite and would like to get all subsites from there.
My JavascriptCode:

currentcontext = new SP.ClientContext.get_current();// ('/sites/subsite1'); 
//    currentweb = currentcontext.get_web();
    site = currentcontext.get_site();
     //.get_serverRelativeUrl().toString();
    currentcontext.load(site);
    web = site.openWebById(subsite11ID);('/sites/subsite1/Subsite11'); 
    currentcontext.load(web);
     
//this.subsites = currentweb.get_webs();
    //this.subsites = currentweb.get_lists();
    this.subsites = web.get_serverRelativeUrl();--> error hier. Function is not initialized

currentcontext.load(this.subsites);
currentcontext.executeQueryAsync(Function.createDelegate(this, this.ExecuteOnSuccess),
Function.createDelegate(this, this.ExecuteOnFailure));

ClientContext accepts only relative Url

anyone can helps me? thanks a lot!