JavaScript undefined vs. null (translation) __java

Accidentally saw a foreigner wrote the discussion JavaScript in undefined and null article, I also learn to translate a back. Find this English is not to rust.

Original English: http://saladwithsteve.com/2008/02/javascript-undefined-vs-null.html

JavaScript undefined vs. null

I was reading a modern, popular book on JavaScript last night and is disappointed by the handling of NULL. The author started out doing a lot to checking like:

Last night, I was reading a JavaScript bestseller, and I was disappointed with the treatment of NULL in the book. The author did a lot of such checks at first:

if (foo = = null) {alert (' foo ' not set. ');}
Then told the reader that they could just remove the = = NULL because JavaScript knows you mean "= null"

Then tell the reader that you can remove "= null" because JavaScript knows that you write "= = null".

what?! This isn ' t why you don ' t check for equality with NULL. It ' s because foo = = null doesn ' t even remotely do what most the IT people in the.

Say what. This is not why you do not check for null. because foo = = NULL is not even indirectly done, in this context many people think it will do (check null).

It ' s a commonly held belief that uninitialized properties in JavaScript are set to NULL as default values. People believe this mostly for 2 reasons:1) foo = = NULL Returns TRUE if Foo is undefined and 2) authors don ' t teach Javas Cript properly.

It is generally assumed that in JavaScript, uninitialized properties are set to NULL by default. This is based on two reasons: 1 if foo is undefined,foo = = NULL Returns TRUE. 2 The author does not teach JavaScript correctly.

A property, where it has no definition, is undefined. Put that way, it ' s pretty obvious.

When an attribute is undefined, it is undefiend. This is a clear case.

Null is an object. It ' s type is null. Undefined is isn't an object, the IT ' s type is undefined. That is less obvious.

Null is an object and its type is null. (Note: This may be a clerical error.) The type of NULL is object. Undefined is not an object and its type is undefined. This part is not very obvious.

The real trouble was that = = does type coercion. = = = Checks for both type and value and are the most intuitive form of equality in JavaScript, in my opinion.

The real problem is that = = do the type conversion. = = = Check both types and values, which I think is the most intuitive form of representation in JavaScript.

I fired up a jash console to hopefully clear things up for you.

I started the Jash console, and I hope I can sort it out for you.

>> Window.hello
Null
>> window.hello.something
Window.hello has no properties
>> Window.hello = null
True
>> Window.hello = = NULL
False
>> Window.hello = = undefined
True
>> if (Window.hello) {alert (' Truthy ');} else {alert (' Falsy ');}/'ll print Falsy.
Null
>> Window.hello = = undefined
True
>> NULL = = undefined
True//There ' s rub, sir.
>> NULL
Null
>> undefined
Null
>> typeof null
Object
>> typeof undefined
Undefined

So people write

if (foo = = null) {
Foo = "Joe";
}
When what they really mean is
if (!foo) {
Foo = "Joe";
}
If you find yourself with a lot of NULL checks in your JavaScript, set aside some time and watch Douglas ' s " JavaScript programming Language "Talk on Yahoo. It ' s Part 1 of a 3-part series of excellent and enlightening talks.

If you find that there are a lot of NULL checks in your JavaScript code, take a moment to look at Douglas Crockford's speech on the JavaScript programming language in Yahoo Video. It is the first part of a series of inspiring and excellent speeches.