Narrow a ternary expression to unique symbol type

[mitar]

🔍 Search Terms

unique symbol
narrowing

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Support creating unique symbols with expressions which always evaluate to the same symbol in runtime.

📃 Motivating Example

I want to use:

export const NONE: unique symbol = process.env.NODE_ENV !== "production" ? Symbol.for("none") : Symbol()

process.env.NODE_ENV does not change during execution. So this will always produce one unique symbol during runtime.

💻 Use Cases

During development, Vite can optimize dependencies and can duplicate imports and thus symbols. So we use Symbol.for to make sure that symbols are deduplicated. Also symbol name is useful for debugging. But in production this is not needed.

Vite can be configured to disable dependency optimizations, but we believe it is better that not all consumers of our code have to do that. Instead, we want to fix this inside the code itself.

[guillaumebrunerie]

During development, Vite can optimize dependencies and can duplicate imports and thus symbols.

So do you mean that any library that depend on global state might just get completely broken during development with Vite? Sounds like a bug in Vite, or am I misunderstanding something?

[mitar]

Problem is that Vite during development optimizes imports in a way that it duplicates them and bundles them together. Which means that this particular import can happen in multiple places, like I can import it with absolute path and with relative path and it could happen that to browser this looks like two different imports because Vite optimizes importing (at cost of duplicating). With symbols, this means that if I import symbol imported from a module relatively it does not match the symbol imported with different import.

See more information here. For example "Vite converts ESM dependencies with many internal modules into a single module to improve subsequent page load performance." So if you happen to load symbol in one of such "converted modules which were combined into a single module" and then at some other point you load it through absolute path, symbols do not match.