javascript check undefined or null

There are two primitive types in javascript: null and undefined. These two types often make JavaScript developers confused, when is null, and when is undefined?The Undefined type has on

In JavaScript, we are often exposed to the 5 more special objects mentioned in the topic--false, 0, empty strings, null, and undefined. These objects are easy to use, so you have to be careful when you use them.Type detectionLet's come down and see what their type is:Running the above code, the dialog box that pops up should display true. That is, False is a Bool

In JavaScript, we are often exposed to the 5 more special objects mentioned in the topic--false, 0, empty strings, null, and undefined. These objects are easy to use, so you have to be careful when you use them.Type detectionLet's come down and see what their type is:　　Running the above code, the dialog box that pops up should display true. That is, False is a Bo

Introduction to null, undefined, and nullundefined in JavaScript Null indicates a special value, which is often used to describe "null ". Perform the typeof operation on null, and the result returns the string "object". That is t

There are 2 special values in JavaScript that represent information that does not exist: null and undefined. The individual believes that the difference between these two special values can be understood from the following perspectives: 1.null represents a container for storing information (such as a variable that w

Say null first, which represents a special value, commonly used to describe "null". Performs a typeof operation on NULL, resulting in the return of the string "object", which means that null can be thought of as a special object value, meaning "Non-object" (Feeling strange). In fact,

One, JavaScript data typeThe data types of JavaScript fall into the following categories:Five simple data types: Undefined,null,boolean,string,number.A complex data type: Object.Second, typeof operatorHere are a few short stories to summarize it, before summing up, first know a very useful operator--typeof.The typeof o

There are two primitive types in javascript: null and undefined. These two types often make JavaScript developers confused, when is null, and when is undefined?The undefined type has on

http://www.cnblogs.com/meil/archive/2006/12/01/578868.html null: indicates no value;Undefined: Represents an undeclared variable,Or a variable that has been declared but not assigned a value,Or an object property that does not exist.The = = operator considers the two equal. If you want to differentiate between the two, use the = = = or typeof operator.Using the IF (!object) {} both containAdded: 2006.12.6va

Undefinedindicates that the variable has been declared but has not been initialized. It should be noted that when the TypeOf operator is used to determine the data type, the values returned by the undeclared and uninitialized variables are undefined var message; Console.log (typeof message),//undefined Console.log (typeof age);//undefined

Null: Null is a javascript keyword, indicating "null". It is a javascript built-in object that does not contain any value. Alert (typeof (null); // The result is an object. When declaring a variable, it can be used as a placehold

Typically, when we try to accesssomething that doesn't exist.Orvariable with no value assigned, you will get aundefinedValue. JavaScript automatically sets the variable that is declared to be not initialized to undifined.What happens if a variable doesn't exist at all:>>> Fooreferenceerror:foo is not definedWe got an error message. We use the typeof operator on this variable to see what type it is:>>> typeof Foo"u