seal

Summary

Prevents the modification of attributes of existing properties, and prevents the addition of new properties.

Syntax

Object.seal( object )

object: Required. The object on which to lock the attributes.

Return Value

The object that is passed to the function.

Examples

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

// Create an object that has two properties.
 var obj = { pasta: "spaghetti", length: 10 };
 // Seal the object.
 Object.seal(obj);
 document.write(Object.isSealed(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);
 document.write("<br/>");

 // Try to delete a property, and then verify that it is still present.
 delete obj.length;
 document.write(obj.length);

 // Output:
 // true
 // undefined
 // 10

Remarks

The Object.seal function does both of the following:

Makes the object non-extensible, so that new properties cannot be added to it.
Sets the configurable attribute to false for all properties of the object.

When the configurable attribute is false , property attributes cannot be changed and the property cannot be deleted. When configurable is false and writable is true , the value and writable attributes can be changed.

The Object.seal function does not change the writable attribute.

For more information about how to set property attributes, see Object.defineProperty Function. To get 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

Exceptions

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

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