ECMA Script Client Object Model Error: The property or field has not been initialized…

I wrote the following code:

function initialize()

{

ctx = new SP.ClientContext.get_current();

currentweb = ctx.get_web();

ctx.load(currentweb, 'Id');

var webid = currentweb.get_id();

ctx.executeQueryAsync(Function.createDelegate(this, onSuccess), Function.createDelegate(this, onFail));

}

Notice the bold and italic line of code that will generate the following error:
Message: The property or field has not been initialized. It has not been requested or the request has not been executed. It may need to be explicitly requested.

Solution:

• Add ExecuteOrDelayUntilScriptLoaded(initialize,'SP.js'); in the code block to ensure the SP.js fully loaded before using it.
• Place var webid = current web.get_id() in the function onSuccess. It seems that you can get the web properties in the same function of it’s being loaded