The difference between "Var" and "Var" is used when declaring "reprint" in JavaScript

Original link: http://www.2cto.com/kf/201204/128406.html "invasion and deletion"

While JavaScript declares variables, while declaring them with the Var keyword and declaring them without the keyword, there are many times when it's not a problem to run, but there's a difference between the two ways.

While JavaScript declares variables, while declaring them with the Var keyword and declaring them without the keyword, there are many times when it is not a problem to run, but there is a difference between the two ways. Code that works correctly does not represent the appropriate code.

var num = 1;

is to declare the variable in the current domain. A local variable (local variable) if declared in the method, or a global variable if declared in the global domain.

and num = 1;

In fact, it is a property assignment operation. First, it tries to chain in the current scope (as declared in the method, then the current scope chain represents the global scope and the method local scope etc ... ), if NUM is found in any current scope chain, the NUM attribute is executed, and if NUM is not found, it will create the NUM attribute and assign a value in the global object (that is, the topmost object of the current scope chain, such as the Window object).

Attention! Instead of declaring a global variable, it creates a property of a global object.

Even so, it may be hard for you to understand the difference between a "variable declaration" and a "Create object property" here. In fact, JavaScript's variable declarations, creation properties, and each of the properties in each JavaScript have a certain flag stating their properties----such as read-only (ReadOnly) non-enumerable (Dontenum) non-deleting (dontdelete), and so on.

Since the variable declaration comes with a non-deleted attribute, the comparison of var num = 1 with num = 1, which is a variable declaration with a non-deleted attribute and cannot be deleted, is a property of the global variable and can therefore be removed from the global variable.

See the following code for details:

Copy CodeThe code is as follows:
NUM1 is a global variable, num2 is a property of window

var num1 = 1;

num2 = 2;

Delete num1; Cannot delete

Delete num2; Delete

function model () {

var num1 = 1; Local variables

num2 = 2; Properties of Window

anonymous functions

(function () {

var num = 1; Local variables

NUM1 = 2; Inheritance scope (closures)

NUM3 = 3; Properties of Window

}())

}

PS. In the ECMASCRIPT5 standard, there is a "strict pattern" (Strict mode). In strict mode, assigning a value to an undeclared identifier throws a reference error, thus preventing accidental creation of global variable properties. Some new versions of browsers are now supported.

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The difference between "Var" and "Var" is used when declaring "reprint" in JavaScript