SharePoint Built-in JS Functions

//Get Current User ID
_spUserIdex:
 alert(_spUserId);

//Run JS Function on Page Load
_spBodyOnLoadFunctionNames.push(' FunctionName ');

//First we must call the EnsureSetup method
JSRequest.EnsureSetup();

//Get a query string parameter called ItemId. i.e - "page.aspx?ItemId=11" will return 11
itemId = JSRequest.QueryString["ItemId"];

//Get the current page name. i.e - "default.aspx"
itemId = JSRequest.FileName;

//Get the current path name. i.e - "/doclib/default.aspx"
itemId = JSRequest.PathName;