TypeError: Cannot create property 'x' on number '1' (V8-based)
TypeError: can't assign to property "x" on 1: not an object (Firefox)
TypeError: Attempted to assign to readonly property. (Safari)

TypeError.

In strict mode, a TypeError is raised when attempting to create a property on primitive value such as a symbol, a string, a number or a boolean. Primitive values cannot hold any property.

The problem might be that an unexpected value is flowing at an unexpected place, or that an object variant of a String or a Number is expected.

Either fix the code to prevent the primitive from being used in such places, or fix the issue by creating the object equivalent Object.

"use strict";

const foo = new String("my string");
foo.bar = {};

• Strict mode
• primitive