Object.isExtensible Function (JavaScript)

2017-1-18 1 minutes to read Contributors

Returns a value that indicates whether new properties can be added to an object.

Syntax

Object.isExtensible(object)

Parameters

object
Required. The object to test.

Return Value

true if the object is extensible, which indicates that new properties can be added to the object; otherwise, false.

Exceptions

If the object argument is not an object, a TypeError exception is thrown.

Remarks

For information about how to set property attributes, see Object.defineProperty Function. To obtain the attributes of a property, you can use the Object.getOwnPropertyDescriptor Function.

The following related functions prevent the modification of object attributes.

Function	Object is made non-extensible	`configurable` is set to `false` for each property	`writable` is set to `false` for each property
Object.preventExtensions	Yes	No	No
Object.seal	Yes	Yes	No
Object.freeze	Yes	Yes	Yes

The following functions return true if all of the conditions marked in the following table are true.

Function	Object is extensible?	`configurable` is `false` for all properties?	`writable` is `false` for all data properties?
`Object.isExtensible`	Yes	No	No
Object.isSealed	No	Yes	No
Object.isFrozen	No	Yes	Yes

Example

The following example illustrates the use of the Object.isExtensible function.

// Create an object that has two properties.  
var obj = { pasta: "spaghetti", length: 10 };  

// Make the object non-extensible.  
Object.preventExtensions(obj);  

// Try to add a new property, and then verify that it is not added.  
obj.newProp = 50;  
document.write(obj.newProp);  

// Output:  
undefined

Requirements

Supported in the following document modes: Internet Explorer 9 standards, Internet Explorer 10 standards, and Internet Explorer 11 standards. Also supported in Store apps (Windows 8 and Windows Phone 8.1). See Version Information.
Not supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards.