TypeError: can't assign to property "x" on "y": not an object
The JavaScript strict mode exception "can't assign to property" occurs when attemptingto create a property onprimitive valuesuch as asymbol, astring, anumber or aboolean.Primitive values cannot hold anyproperty.
Message
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)
Error type
What went wrong?
Instrict mode, aTypeError is raised when attempting tocreate a property onprimitive value suchas asymbol, astring, anumber or aboolean.Primitive values cannot hold anyproperty.
The problem might be that an unexpected value is flowing at an unexpected place, orthat an object variant of aString or aNumber is expected.
Examples
Invalid cases
js
"use strict";const foo = "my string";// The following line does nothing if not in strict mode.foo.bar = {}; // TypeError: can't assign to property "bar" on "my string": not an object