Implementing Private Fields for JavaScript

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2021-06-08 17:30:04

When implementing a language feature for JavaScript, an implementer must make decisions about how the language in the specification maps to the implementation. Sometimes this is fairly simple, where the specification and implementation can share much of the same terminology and algorithms. Other times, pressures in the implementation make it more challenging, requiring or pressuring the implementation strategy diverge to diverge from the language specification.

Private fields is an example of where the specification language and implementation reality diverge, at least in SpiderMonkey-- the JavaScript engine which powers Firefox. To understand more, I'll explain what private fields are, a couple of models for thinking about them, and explain why our implementation diverges from the specification language.

Private fields are a language feature being added to the JavaScript language through the TC39 proposal process, as part of the class fields proposal, which is at Stage 4 in the TC39 process. We will ship private fields and private methods in Firefox 90.