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 14. Null
Conceptual Overview of Using the null Value
You can use null to explicitly
indicate that an object property does not contain a value. Typically, if a
property is set up to contain a value, but the value is not available for
some reason, the value null should be
used to indicate that the reference property has an empty value.
<!DOCTYPE html><html lang="en"><body><script> // the property foo is waiting for a value, so we set its initial value to null var myObjectObject = {foo: null}; console.log(myObjectObject.foo); //logs 'null' </script></body></html>
Note
Don’t confuse null with
undefined.
undefined is used by JavaScript to tell you that
something is missing. null is provided so you can
determine when a value is expected but just not available yet.
typeof Returns null Values as “object”
For a variable that has a value of null, the typeof operator returns
'object'. If you need to verify a null value, the ideal solution would be to see
if the value you are after is equal to null. Below, we use the === operator to specifically verify that we are
dealing with a null value.
<!DOCTYPE html><html lang="en"><body><script> var myObject = null; console.log(typeof myObject); // logs 'object', not exactly helpful console.log(myObject === null); // logs true, only for a real null value </script></body></html>
Note
When verifying a null value,
always use === because == does not distinguish between null and 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.
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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.