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Message
TypeError: property "x" is non-configurable and can't be deleted. (Firefox) TypeError: Cannot delete property 'x' of #<Object> (Chrome)
Error type
TypeError
in strict mode only.
What went wrong?
It was attempted to delete a property, but that property is non-configurable. The 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
.
Examples
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