preventExtensions

Summary

Prevents the addition of new properties to an object.

Syntax

Object.preventExtensions( object )

object: Required. The object to make non-extensible.

Return Value

The object that is passed to the function.

Examples

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

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

 // Make the object non-extensible.
 Object.preventExtensions(obj);
 document.write(Object.isExtensible(obj));
 document.write("<br/>");

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

 // Output:
 // false
 // undefined

Remarks

The Object.preventExtensions function makes an object non-extensible, so that new named properties cannot be added to it. After an object is made non-extensible, it cannot be made extensible.

For information about how to set property attributes, see Object.defineProperty 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

Exceptions

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

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