Remove older emit support over time

[JoshuaKGoldberg]

Suggestion

🔍 Search Terms

remove emit target es3 es5

✅ Viability Checklist

My suggestion meets these guidelines:

• 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 feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Following #47572, once the default target has been raised to a new value, can TypeScript remove emit support for the no-longer-default old version in either that same major version or the next one?

Using the example schedule in that issue:

• Remove es3 emit support for 5.0, making es5 the lowest supported emit target
• Remove es5 emit support for 6.0, making es2015 the lowest supported emit target
• ...and so on

📃 Motivating Example

Supporting emit to low emit targets such as ES3/5 worsens:

• Dev cost for some TypeScript features: e.g. Allowed non-this, non-super code before super call in derived classes with property initializers #29374's changes were mostly in emit
• Output package size: e.g. src/compiler/transformers/es2015.ts has >4k lines today (about 10% the size of src/compiler/checker.ts)

💻 Use Cases

It'd be nice to have a leaner TypeScript without the cost of supporting emit for syntax compatibility the vast majority of runtimes no longer need.

[RyanCavanaugh]

We removed ES3 support in 5.0. ES5 for 6.0 is certainly a possibility.