メッセージ
TypeError: can't assign to property "x" on {y}: not an object (Firefox)
TypeError: Cannot create property 'x' on {y} (Chrome)
エラータイプ
何がうまくいかなかったのか？
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.
例
無効なケース
'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
問題を修正する
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 = {};