December 2012
Beginner
164 pages
3h 15m
English
Content preview from JavaScript EnlightenmentBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Chapter 15. Undefined
Conceptual Overview of the undefined Value
The undefined value is used by
JavaScript in two slightly different ways.
The first way it’s used is to indicate that a declared variable
(e.g., var foo) has no
assigned value. The second way it’s used is to
indicate that an object property you’re trying to access is not
defined (i.e., it has not even been named), and is
not found in the prototype chain.
Below, I examine both usages of undefined by JavaScript.
<!DOCTYPE html><html lang="en"><body><script> var initializedVariable; // declare variable console.log(initializedVariable); // logs undefined console.log(typeof initializedVariable); / confirm that JavaScript returns undefined / var foo = {}; console.log(foo.bar); // logs undefined, no bar property in foo object console.log(typeof foo.bar); // confirm that JavaScript returns undefined </script></body></html>
Note
It is considered good practice to allow JavaScript alone to use
undefined. You should never find
yourself setting a value to undefined, as in foo =
undefined. Instead, null should be used if you are specifying that
a property or variable value is not available.
JavaScript ECMAScript 3 Edition (and Later) Declares the undefined Variable in the Global Scope
Unlike previous versions, JavaScript ECMAScript 3 Edition (and
later) has a global variable called undefined declared in the global scope. Because
the variable is declared, and not assigned a value, the undefined variable
is set to undefined.
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.