TypeError: can't assign to property "x" on "y": not an object
The JavaScript strict mode exception "can't assign to property" occurs 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.
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?
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.
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