The JavaScript exception "property is non-configurable and can't be deleted" occurs when it was attempted to delete a property, but that property is non-configurable .
TypeError: Calling delete on 'x' is not allowed in strict mode (Edge) TypeError: property "x" is non-configurable and can't be deleted. (Firefox) TypeError: Cannot delete property 'x' of #<Object> (Chrome)
TypeError
in strict mode only.
It was attempted to delete a property, but that property is
non-configurable
。
configurable
attribute controls whether the property can be deleted from the object and whether its attributes (other than
writable
) can be changed.
This error happens only in
strict mode code
. In non-strict code, the operation returns
false
.
Non-configurable properties are not super common, but they can be created using
Object.defineProperty()
or
Object.freeze()
.
'use strict';
var obj = Object.freeze({name: 'Elsa', score: 157});
delete obj.score; // TypeError
'use strict';
var obj = {};
Object.defineProperty(obj, 'foo', {value: 2, configurable: false});
delete obj.foo; // TypeError
'use strict';
var frozenArray = Object.freeze([0, 1, 2]);
frozenArray.pop(); // TypeError
There are also a few non-configurable properties built into JavaScript. Maybe you tried to delete a mathematical constant.
'use strict'; delete Math.PI; // TypeError