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.

TypeError: can't assign to property "x" on {y}: not an object (Firefox)
TypeError: Cannot create property 'x' on {y} (Chrome)

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.

'use strict';

var 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

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

'use strict';

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

• Strict_mode
• primitive