Respect the default setting of the useDefineForClassFields tsconfig option

[kfdf]

Sometime between 0.11.20 and 0.12.6 esbuild stopped transpiling public class fields into assignments inside the constructor when the target tsconfig setting is set to esnext and useDefineForClassFields is not present. This breaks LitElement decorators that make properties reactive, as described here. It's mentioned there that useDefineForClassFields is set to false by default. Maybe the default value should be respected when transpiling typescript files?

[evanw]

The TypeScript language now works like this as of TypeScript 4.3.1, so esbuild has been changed to match. The issue microsoft/TypeScript#34787 has some additional context. I'm not sure why the TypeScript team decided to release this as a silent breaking change without an announcement. But the documentation for LitElement is now incorrect. The default value is false except when the target is "ESNext" in which case the default is true.

Anyway specifying "target": "ESNext" in your tsconfig.json file without also setting a value for "useDefineForClassFields" is now error-prone because it means your code will now behave differently in different versions of TypeScript. I personally think the TypeScript compiler should warn when you do this and tell you to set "useDefineForClassFields" explicitly.

If you are using an old version of TypeScript you can either add "useDefineForClassFields": false in your tsconfig.json file or, if you really don't want to change tsconfig.json, you can also just use an old version of esbuild.

[kfdf]

Oh, I see.