无法立即调用onKeyPress函数

Question

  document.getElementById('my').onkeypress = function(event) { this.value = this.value.toUpperCase() } 

  <input id='my' type="text"> 

Can someone explain to me what the this variable points to in the code above? Does it point to the input html element I grabbed from the document.getElementById('my) ? Or does it point to the window object in the browser? Also what is the event parameter passed into the anonymous function?

For some reason when I run this code the value of each character is uppercased except the last char. So for example when I enter: a. The character a is not uppercased immediately it is only uppercased when I enter another letter. For example: Ab. Now b is not uppercased until I enter another letter after b: ABc. And this continues on. Can someone please explain why this happen?

Answer 1

Can someone explain to me what the this variable points to in the code above? Does it point to the input html element I grabbed from the document.getElementById('my)? Or does it point to the window object in the browser? Also what is the 'event' parameter passed into the anonymous function?

• this refers to the input element
• it's not an anonymous function, it's the declaration of the onkeypress function
• event is a parameter of the above function more info

The unexpected(?) behaviour is because the code is executed before the input field is updated, so only the previous value gets uppercased. To avoid it, you can use onkeyup event which fires after you release a key.

See it here:

 document.getElementById('my').onkeypress = function(event) { this.value = this.value.toUpperCase() } document.getElementById('my2').onkeyup = function(event) { this.value = this.value.toUpperCase() } document.getElementById('my3').oninput = function(event) { this.value = this.value.toUpperCase() } 

 <input id='my' type="text" placeholder="onkeypress"> <input id='my2' type="text" placeholder="onkeyup"> <input id='my3' type="text" placeholder="oninput"> 

Thanks to @www139 for bringing up oninput in the comments as it is indeed the best solution for modern browsers Please note it's HTML5-only and you can't get info like the key code, identifier, etc...

Answer 2

Here is a simple definition of the key events references from w3c.

Tip: The order of events related to the onkeypress event:

onkeydown onkeypress onkeyup Note: The onkeypress event is not fired for all keys (eg ALT, CTRL, SHIFT, ESC) in all browsers. To detect only whether the user has pressed a key, use onkeydown instead, because it works for all keys.

This explains the oninput event; http://help.dottoro.com/ljhxklln.php

The oninput event has support of all the major browsers including IE 9 and later.

oninput better suits your needs than the "second-best" onkeyup because onkeyup only fires when the key is released. When you type, the new value is entered as soon as it is pressed, however it seems to add the new value to the box after the function fires. I will continue to experiment because I have another idea of how to do this but oninput seems like the best key event for this at the moment.

 document.getElementById('my').oninput = function(event) { this.value = this.value.toUpperCase() } 

 <input id='my' type="text"> 

UPDATE

You can still actually use the onkeypress event but you need to use a timeout to wait for the new value to be inserted into the text box before executing the function.

Here is another code snippet....

 document.getElementById('my').onkeypress = function(event) { var elem = this; window.setTimeout(function() { elem.value = elem.value.toUpperCase() }, 0); } 

 <input id='my' type="text">