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