The usual way to check if the value of a property is the special value undefined
, is:
if(o.myProperty === undefined) {
alert("myProperty value is the special value `undefined`");
}
To check if an object does not actually have such a property, and will therefore return undefined
by default when you try and access it:
if(!o.hasOwnProperty('myProperty')) {
alert("myProperty does not exist");
}
To check if the value associated with an identifier is the special value undefined
, or if that identifier has not been declared. Note: this method is the only way of referring to an undeclared (note: different from having a value of undefined
) identifier without an early error:
if(typeof myVariable === 'undefined') {
alert('myVariable is either the special value `undefined`, or it has not been declared');
}
In versions of JavaScript prior to ECMAScript 5, the property named "undefined" on the global object was writeable, and therefore a simple check foo === undefined
might behave unexpectedly if it had accidentally been redefined. In modern JavaScript, the property is read-only.
However, in modern JavaScript, "undefined" is not a keyword, and so variables inside functions can be named "undefined" and shadow the global property.
If you are worried about this (unlikely) edge case, you can use the void operator to get at the special undefined
value itself:
if(myVariable === void 0) {
alert("myVariable is the special value `undefined`");
}
2015 update: the flexbox approach
There are two other answers briefly mentioning flexbox; however, that was more than two years ago, and they don't provide any examples. The specification for flexbox has definitely settled now.
Note: Though CSS Flexible Boxes Layout specification is at the Candidate Recommendation stage, not all browsers have implemented it. WebKit implementation must be prefixed with -webkit-; Internet Explorer implements an old version of the spec, prefixed with -ms-; Opera 12.10 implements the latest version of the spec, unprefixed. See the compatibility table on each property for an up-to-date compatibility status.
(taken from https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Flexible_boxes)
All major browsers and IE11+ support Flexbox. For IE 10 or older, you can use the FlexieJS shim.
To check current support you can also see here:
http://caniuse.com/#feat=flexbox
Working example
With flexbox you can easily switch between any of your rows or columns either having fixed dimensions, content-sized dimensions or remaining-space dimensions. In my example I have set the header to snap to its content (as per the OPs question), I've added a footer to show how to add a fixed-height region and then set the content area to fill up the remaining space.
html,
body {
height: 100%;
margin: 0;
}
.box {
display: flex;
flex-flow: column;
height: 100%;
}
.box .row {
border: 1px dotted grey;
}
.box .row.header {
flex: 0 1 auto;
/* The above is shorthand for:
flex-grow: 0,
flex-shrink: 1,
flex-basis: auto
*/
}
.box .row.content {
flex: 1 1 auto;
}
.box .row.footer {
flex: 0 1 40px;
}
<!-- Obviously, you could use HTML5 tags like `header`, `footer` and `section` -->
<div class="box">
<div class="row header">
<p><b>header</b>
<br />
<br />(sized to content)</p>
</div>
<div class="row content">
<p>
<b>content</b>
(fills remaining space)
</p>
</div>
<div class="row footer">
<p><b>footer</b> (fixed height)</p>
</div>
</div>
In the CSS above, the flex property shorthands the flex-grow, flex-shrink, and flex-basis properties to establish the flexibility of the flex items. Mozilla has a good introduction to the flexible boxes model.
Best Answer
Slight changes over earlier version - tested on CHROME