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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