Develop advanced tutorials Using Dojo's AJAX applications, part 1th

With the popularity of Ajax applications, the JavaScript language gets more and more attention. Developers are also increasingly using JavaScript. JavaScript is no longer just used to add some fancy effects to the page, it has become an important cornerstone in building Ajax applications. JavaScript, as a dynamic language specifically designed to execute in a browser, has many important features and differs from traditional Java or C + + languages. Familiarity with these features can help developers better develop Ajax applications. This article describes 13 of the more important features in the JavaScript language, including prototype, execution context, scope chain, and closures.

null and undefined

There are 5 basic types in JavaScript, namely String, number, Boolean, Null, and Undefined. The first 3 are better understood, and the back two kinds are slightly more complex. A null type has only one value, or null, and the Undefined type has only one value, that is, Undefined. Both null and undefined can be used directly in JavaScript code as literal (literal).

Null is associated with an object reference, expressed as an empty or nonexistent object reference. When a variable is declared without assigning a value to it, its value is undefined.

The value of the undefined will appear in the following situations:

Gets a property from an object that has a value of undefined if none of the objects in the object and its prototype chain have that property.

A function that returns a value to its caller without an explicit return value is undefined. One special case is when you are using new.

A function in JavaScript can declare any form parameter, and when the function is actually called, the number of arguments passed in is less than the form parameter of the declaration, and the value of the extra formal parameter is undefined.

There are some interesting features about null and undefined:

If you use the typeof operator for a variable with a null value, the result is object, and the undefined value is typeof, and the result is undefined. such as typeof null = = "Object"; typeof undefined = = "undefined"

Null = = undefined, but null!== undefined

if ("" | | 0)

For an if expression, everyone is not unfamiliar. The true and false values of the expressions immediately following the IF in JavaScript are judged to be different from other languages. Please see table 1 for details.

Table 1. True and false values in JavaScript

Type	True or False value
Null	Always False (false)
Undefined	Always False (false)
Boolean	Keep the true and false values unchanged
Number	+0,-0 or NaN is false, other values are true
String	Empty string is false, other value is true
Object	Always True (true)

As you can see in table 1, the values in JavaScript that make if judgment false may have null, undefined, false, +0,-0, NaN, and Empty strings ("").