javascript check undefined or null

There are two original types in javascript: null and undefined. These two types often make JavaScript developers wonder when they are null and when are undefined? The undefined type ha

Here is a collection of JS in the null and undefined two hints of the difference between analysis, for each beginner should have no small use OH. Generally known:Null = = undefinedBut:Null!== undefinedSo what's the difference between the two?NullThis is an object, but is empty. Because it is an object, typeof null returns ' object '.

It is hard to answer at a time, especially undefined, because it involves the implementation principle of undefined. So, after thinking about it, write down this article. Please take a look.Total offices: null = undefinedBut: null! = UndefinedSo what is the difference between the two?Please listen to me again...

It is hard to answer at a time, especially undefined, because it involves the implementation principle of undefined. So, after thinking about it, write down this article. Please take a look.Total offices: null = undefinedBut: null! = UndefinedSo what is the difference between the two?Please listen to me again...

In Javascript, two values are used to represent concepts similar to null values, undefined and null, which are easily obfuscated and represent two different concepts. Similar articles have not been posted before. Difference between JavaScript

A bad time to answer, especially undefined, because this involves the realization of the principle of undefined. So, after thinking carefully, write down this article, please heroes to shoot bricks.Total known: null = = undefinedHowever: null!== undefinedSo what's the difference between the two?Please listen to me expl

above code can conclude that: (1) undefined is equal to NULL, (2) Nan is not equal to any value, nor is it equal to itself. JavaScript Undefined PropertiesDefinition and usageThe undefined property is used to store JavaScript

When looking for some JavaScript Information, I read some blog posts, one of which is a blog http://blog.csdn.net/leadzen/article/details/3899392 about the analysis of null and undefined, which summarizes some, comments Let me think a lot, I personally think it is an article with a lot of misunderstanding, but the visits and replies are basically good. I even sus

I believe many beginner JavaScript will have this idea: why JavaScript is null and undefined, relative to C, C + +, Java, they are all just null, why JavaScript is differentOne, historical reasons The web search learned that,when

Difference between Null and Undefined in JavaScript This article mainly introduces the difference between Null and Undefined in JavaScript. This article provides multiple code instances to explain the differences between them. For

How to Judge-null js in JS How to judge-undefined JS how to judge-string type of number The following is an incorrect method: var exp = undefined;if (exp = = undefined){Alert ("undefined");} When exp is null, it also gets the sam

Null: indicates no value;Undefined: indicates an undeclared variable,Or a variable that has been declared but has no value assigned,Or an object attribute that does not exist. The = Operator treats the two as equal. To distinguish the two, use the = or typeof operator.Use if (! Object) {} both contain Supplement: 2006.12.6VaR OBJ = "AAA ";VaR nullobj; If (OBJ = NU