TypeError: Cannot delete property 'x' of #<Object> (V8-based)
TypeError: property "x" is non-configurable and can't be deleted (Firefox)
TypeError: Unable to delete property. (Safari)

TypeError in strict mode only.

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.

Non-configurable properties are not super common, but they can be created using Object.defineProperty() or Object.freeze().

"use strict";
const obj = Object.freeze({ name: "Elsa", score: 157 });
delete obj.score; // TypeError

"use strict";
const obj = {};
Object.defineProperty(obj, "foo", { value: 2, configurable: false });
delete obj.foo; // TypeError

"use strict";
const 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

• delete
• Object.defineProperty()
• Object.freeze()