javascript - "Waiting" for ReadyState to be 4 before returning a value -

i have looked in lots of places this, , i'm being idiot because done lot, here's situation.

i trying display checkbox next e-mail field on website iff e-mail has not been used register already.

what have this:

$('[name=reg_email]').change( function() {      if(!emailused()) {         //update image green checkmark     } else {         //update image huge red x     } } 

and "emailused" function should returning javascript boolean variable depending on whether or not e-mail address in database. facilitate this, i've created script determine if it's there or not. emailused() function needs call script , return, need wait until readystate == 4 before return, , of methods have found waiting readystate equal 4 prevent me returning value @ all:

function emailused() {     var req = $.get('scripts/email_used.php?email='+$('[name=reg_email]').val());      //wait req.readystate 4      return req.responsetext == 'true'; } 

but can find explains how , still return value. seems use callback functions , , that, can't method return value.

help, please!

doing busy wait req.readystate === 4 considered bad design practice. you're tying ui thread , preventing browser updating. if server slow enough respond, user prompted whether cancel further processing.

if take @ $.get(), takes completion function 1 of arguments. should perform success/failure logic in function. can disabling submit button until success message.