Second, strict mode prohibits the use of keywords which are likely to be defined in future versions of ECMAScript, such as ‘implements’, ‘private’, and ‘protected’.
When you start a new application, you can switch on strict mode from the beginning.
You should, however, be careful when using strict mode in applications with an existing codebase. Since some of the errors are only detected during runtime, switching to strict mode for your existing productive code without testing everything can be risky. If you cannot ensure all code is touched by automated tests, you might not want to enable strict mode for your productive code. Instead you can use the strict mode for executing your unit tests to detect some bugs earlier without putting strict mode in the productive implementation.