javascript: how to check a property is undefined

Question

I have codes below.

elem.onkeypress=function(e){
 if( (e.which===undefined?e.keyCode:e.which)==13 ){
   //dosomething
  }
}

at IE8, it occus erros: 'which' is null or not an object

how to fix this problem.

possible duplicate of [Best way to check for "undefined" in javascript](http://stackoverflow.com/questions/3390396/best-way-to-check-for-undefined-in-javascript) and others... — Felix Kling, Oct 21 '10 at 08:48
a little bit diffrence is this one is check wheather a property of a Object is undefined, not a variable. — guilin 桂林, Oct 21 '10 at 09:12
The problem's nothing to do with the `undefined` check of the `which` property. The problem is that `e` is undefined. — Tim Down, Oct 21 '10 at 09:14

Tim Down · Accepted Answer · 2010-10-21T09:30:20.277

5

The problem is that e is undefined in IE because no event object is passed as a parameter to the event handler. You need the window.event property:

elem.onkeypress=function(e) {
  e = e || window.event;
  var charCode = e.which || e.keyCode;
  if (charCode == 13) {
    //dosomething
  }
};

edited Oct 21 '10 at 09:30

answered Oct 21 '10 at 09:12

Tim Down

306,503
71
443
520

score 1 · Answer 2 · answered Oct 21 '10 at 08:50

1

One option is to go with (e.hasOwnProperty('which') ? ...

answered Oct 21 '10 at 08:50

Saul

17,666
8
59
87

score 0 · Answer 3 · answered Oct 21 '10 at 08:48

0

use typeof:

if (typeof e.which == 'undefined' ? e.keyCode : e.which) == 13)

answered Oct 21 '10 at 08:48

Dan Grossman

50,627
10
109
97

javascript: how to check a property is undefined

3 Answers3