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

TypeError.

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

'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

Fixing the issue

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 = {};