Object.isSealed Function (JavaScript)

2017-1-18 2 min to read Contributors

Returns true if existing property attributes cannot be modified in an object and new properties cannot be added to the object.

Syntax

Object.isSealed(object)

Parameters

object
Required. The object to test.

Return Value

true if both of the following are true:

The object is non-extensible, which indicates that new properties cannot be added to the object.
The configurable attribute is false for all existing properties.

If the object does not have any properties, the function returns true if the object is non-extensible.

Exceptions

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

Remarks

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

The Object.isSealed function does not use the writable attribute of properties to determine its return value.

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.isSealed function.

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

// Seal the object, and verify that it is sealed.  
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

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.