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Object.isExtensible Function (JavaScript)

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

Object.isExtensible(object)

object

Required. The object to test. This can be a native JavaScript object or a DOM object.

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

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

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

Related Functions

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?

Object is non-extensible?

configurable is false for all properties?

writable is false for all data properties?

Object.isExtensible

X

Object.isSealed

X

X

Object.isFrozen

X

X

X

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

function showPreventExtensions() {

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

    // Show the extensible attribute of the object.
    showAttribute(obj);

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

    // Show the extensible attribute again.
    showAttribute(obj);

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

function showAttribute(obj) {
    var newLine = "<br />";

    document.write("extensible: " + Object.isExtensible(obj));
    document.write(newLine);
}

Following is the output.

extensible: true
extensible: false
true

Supported in the Internet Explorer 9 standards document mode. See Version Information.

Not supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards.

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