Object.seal Function (JavaScript)

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

Object.seal(object)
object

Required. The object on which to lock the attributes. This can be a native JavaScript object or a DOM object.

The object that is passed to the function.

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

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 (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.seal function.

function showSeal() {

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

    // Show attributes of the object and its properties.
    showAttributes(obj);

    // Seal the object.
    Object.seal(obj);

    // Show the attributes again.
    showAttributes(obj);

    // Try to add a new property, and then verify that it is
    // not added. It is not added because Object.seal makes the
    // object non-extensible.
    obj.newProp = 50;
    document.write(typeof (obj.newProp) === "undefined");
    document.write(" ");

    // Try to delete a property, and then verify that it is
    // still present. It is not deleted because Object.seal sets
    // the configurable attribute of the property to false.
    delete obj.length;
    document.write(obj.length === 10);
}

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

    // Show attributes of the object.
    document.write("non-extensible: " + !Object.isExtensible(obj));
    document.write(", isSealed: " + Object.isSealed(obj));
    document.write(", isFrozen: " + Object.isFrozen(obj));
    document.write(newLine);

    // Show attributes of each property.
    for (var prop in obj) {
        var descriptor = Object.getOwnPropertyDescriptor(obj, prop);

        document.write(prop + " - ");
        document.write("configurable: " + descriptor.configurable);
        document.write(", writable: " + descriptor.writable);
        document.write(newLine);
    }
    document.write(newLine);
}

The following is the output.

non-extensible: false, isSealed: false, isFrozen: false
pasta - configurable: true, writable: true
length - configurable: true, writable: true

non-extensible: true, isSealed: true, isFrozen: false
pasta - configurable: false, writable: true
length - configurable: false, writable: true

true 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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