javascript - Check input when text is changed -

I have a question about when input text should be replaced, I use this as the keydown :

  $ ('# query'). Keydown (function () {console.log ($ (this) .val ());});   
 but the actual value is  query , then the console log will be  quer . This is always a delay, I do not know why?   
 
 
 There are 3 ways to apply it.  
 The first is using the other events like  input  or  propertychange . Note that  propertychange  is only supported on IE and  input  is supported on modern browsers.  
  $ ('# query'). 'Input perpertychange', function () {console.log ($ (this) .val ());});   
 The other way is to  console.log ($ (this) .val ());  in  setTimeout  statement. You can get what you are typing. (Reason? I'm sorry, I do not really know why, use it);  
  $ ('# query'). Keydown (function () {var query = $ (this); settimeout (function () {console.log (query.val ());}, 0);});   
 and the last time using  textchange  events like this:  
  $ ('# query'). ('Textchange', function () {console.log ($ (this) .val ());});