Recommendations for ECMAScript Fifth Edition Implementors

The members of TC39 recommend these items as best practices that implementors should follow in implementing ES5 and extending it. Following these best practices will improve interoperability among ES5 implementations and minimize the risk of that an extension will conflict with a feature of the next edition of the ECMAScript.

The following best practices have been reviewed and approved by TC39. TC39 will make its best possible effort to not introduce any changes into the next edition of EcmaScript that is inconsistent with these recommendations.

• Strict mode declaration extensions: Do not extend strict mode with declarations or statments using the keywords const or let, or allow function declarations to occur in strict mode statement contexts.
• Make non-standard properties configurable: Chapter 16 allows implementations to add non-standard properties to built-in native objects. Like standard native properties, it is desirable for non-standard properties to start out configurable, so that initialization scripts can replace or remove them.

The following best practices are proposed for review by TC39. They may or may not become Approved Best Practices.

• When implementing ES5 make sure that strict mode is the last feature enabled, and that suppression of this-coercion is the last feature of strict mode enabled. This allows simpler detection of claimed ES5 conformance.