<form action=»foo»>
<input type=»text» onkeypress=»return bar(event)»>
<input type=»submit»>
</form>

<script type=»text/javascript»>
function bar(evt){
var k=evt.keyCode||evt.which;
return k!=13;
}
</script>

Note however that this «enter» key behavior isn’t standard (and actually behaves differently across user agents); moreover this won’t prevent you from full server-side checking, as a client can have javascript deactivated (or even trick your javascript at runtime).

You just have to make sure to use it on an associative array and not an object.
Example below

Good (assuming Object has not been overloaded or extended):

var array = new Object(); //this is safe only if you can assure object has not been extended.
//use the below if Object has been extended
var array;

//everything below here works fine regardless of the two above cases
array['something'] = ‘yay’;
array['somethingelse'] = ‘more’;

for ( keyVar in array ) {
alert(array[keyVar]);
}

Bad:

var array = new Array(); //uh oh, array is an object which means it’s an associative array

//the below loop will actually give you results, despite never putting «values» into
//the array variable. This is because array has functions defined from the Array() object
//which will be grabbed by the keyVar since objects are really associative arrays
for ( keyVar in array ) {
alert(array[keyVar]);
}