javascript Effect of declared and undeclared variables

Effect of declared and undeclared variables

提问by Maizere Pathak.Nepal

What is the major difference between JavaScript declared and undeclared variables, since the delete operator doesn't work on declared variables?

What is the major difference between JavaScript declared and undeclared variables, since the delete operator doesn't work on declared variables?

 var y = 43;     // declares a new variable
 x = 42;

 delete x;       // returns true  (x is a property of the global object and can be deleted)
 delete y;       // returns false (delete doesn't affect variable names) 

Why does this happen? Variables declared globally are also the properties of the window object, so why can't it be deleted?

Why does this happen? Variables declared globally are also the properties of the window object, so why can't it be deleted?

采纳答案by Matt Coughlin

Declared and undeclared global variables

Declared and undeclared global variables

The mechanism for storing and accessing them is the same, but JavaScript treats them differently in some cases based on the value of the configurableattribute (described below). In regular usage, they should behave the same.

The mechanism for storing and accessing them is the same, but JavaScript treats them differently in some cases based on the value of the configurableattribute (described below). In regular usage, they should behave the same.

Both exist in the global object

Both exist in the global object

Below are some comparisons of declaredand undeclaredglobal variables.

Below are some comparisons of declaredand undeclaredglobal variables.

var declared = 1;  // Explicit global variable (new variable)
undeclared   = 1;  // Implicit global variable (property of default global object)

window.hasOwnProperty('declared')    // true
window.hasOwnProperty('undeclared')  // true

window.propertyIsEnumerable('declared')    // true
window.propertyIsEnumerable('undeclared')  // true

window.declared     // 1
window.undeclared   // 1

window.declared   = 2;
window.undeclared = 2;

declared     // 2
undeclared   // 2

delete declared     // false
delete undeclared   // true
delete undeclared   // true (same result if delete it again)

delete window.declared     // false
delete window.undeclared   // true (same result if delete it yet again)
delete window.undeclared   // true (still true)

Both declaredand undeclaredglobal variables are properties of the windowobject (the default global object). Neither one is inherited from a different object through the prototype chain. They both exist directly in the windowobject (since window.hasOwnPropertyreturns truefor both).

Both declaredand undeclaredglobal variables are properties of the windowobject (the default global object). Neither one is inherited from a different object through the prototype chain. They both exist directly in the windowobject (since window.hasOwnPropertyreturns truefor both).

The configurable attribute

The configurable attribute

For declaredglobal variables, the configurableattribute is false. For undeclaredglobal variables, it's true. The value of the configurableattribute can be retrieved using the getOwnPropertyDescriptormethod, as shown below.

For declaredglobal variables, the configurableattribute is false. For undeclaredglobal variables, it's true. The value of the configurableattribute can be retrieved using the getOwnPropertyDescriptormethod, as shown below.

var declared = 1;
undeclared = 1;

(Object.getOwnPropertyDescriptor(window, 'declared')).configurable     // false
(Object.getOwnPropertyDescriptor(window, 'undeclared')).configurable   // true

If the configurableattribute of a property is true, the attributes of the property can be changed using the definePropertymethod, and the property can be deleted using the deleteoperator. Otherwise, the attributes cannot be changed, and the property cannot be deleted in this manner.

If the configurableattribute of a property is true, the attributes of the property can be changed using the definePropertymethod, and the property can be deleted using the deleteoperator. Otherwise, the attributes cannot be changed, and the property cannot be deleted in this manner.

In non-strict mode, the deleteoperator returns trueif the property is configurable, and returns falseif it's non-configurable.

In non-strict mode, the deleteoperator returns trueif the property is configurable, and returns falseif it's non-configurable.

Summary

Summary

Declared global variable

Declared global variable

• Is a property of the default global object (window)
• The property attributes cannotbe changed.
• Cannotbe deleted using the deleteoperator

• Is a property of the default global object (window)
• The property attributes cannotbe changed.
• Cannotbe deleted using the deleteoperator

Undeclared global variable

Undeclared global variable

• Is a property of the default global object (window)
• The property attributes canbe changed.
• Canbe deleted using the deleteoperator

• Is a property of the default global object (window)
• The property attributes canbe changed.
• Canbe deleted using the deleteoperator

See also

See also

• delete operator
• Object.defineProperty
• Object.getOwnPropertyDescriptor
• hasOwnProperty
• Strict mode

• delete operator
• Object.defineProperty
• Object.getOwnPropertyDescriptor
• hasOwnProperty
• Strict mode

回答by Sasank Sunkavalli

When any variable is created via Variable Declaration in JavaScript, these properties are created with "DontDelete" attribute , which basically means that variable you created cannot be Deleted using "delete" expression. All the functions, arguments , function parameters by default are created with this DontDelete attribute. You can think of DontDelete as a flag.

When any variable is created via Variable Declaration in JavaScript, these properties are created with "DontDelete" attribute , which basically means that variable you created cannot be Deleted using "delete" expression. All the functions, arguments , function parameters by default are created with this DontDelete attribute. You can think of DontDelete as a flag.

var y = 43;
delete y;         //returns false because it is has a DontDelete attribute

Whereas Undeclared assignment doesn't set any attributes like DontDelete. So when we apply deleteoperator on this undeclared variable , it returns true.

Whereas Undeclared assignment doesn't set any attributes like DontDelete. So when we apply deleteoperator on this undeclared variable , it returns true.

x = 42;
delete x;        //returns true because it doesn't have a DontDelete attribute

The difference between property assignment and variable declaration — latter one sets DontDelete, whereas former one doesn't. That's why undeclared assignment creates a deletable property.

The difference between property assignment and variable declaration — latter one sets DontDelete, whereas former one doesn't. That's why undeclared assignment creates a deletable property.

Link on how exactly delete operator works

Link on how exactly delete operator works

回答by Pat

The main difference is when you're declaring variables inside a function. If you use varwhen you're declaring a variable inside a function, then that variable becomes a local variable. However, if you don't use var, then the variable becomes a global variable no matter where you declare it (inside or outside a function).

The main difference is when you're declaring variables inside a function. If you use varwhen you're declaring a variable inside a function, then that variable becomes a local variable. However, if you don't use var, then the variable becomes a global variable no matter where you declare it (inside or outside a function).

回答by Jay Bhatt

delete is only effective on an object's properties. It has no effect on variable or function names.

delete is only effective on an object's properties. It has no effect on variable or function names.

In your case x = 42; declares variable X and makes it the property of the Global object. So it returns true.

In your case x = 42; declares variable X and makes it the property of the Global object. So it returns true.

And var y = 43; declares a global variable which is not part of any object so it returns false.

And var y = 43; declares a global variable which is not part of any object so it returns false.