Merge pull request #2127 from jablko/patch-15

Document const enum pitfalls and workarounds

Author: Orta Therox

packages/documentation/copy/en/reference/Enums.md

@@ -365,6 +365,33 @@ let directions = [
 ];
 ```
 
+#### Const enum pitfalls
+
+Inlining enum values is straightforward at first, but comes with subtle implications.
+These pitfalls pertain to _ambient_ const enums only (basically const enums in `.d.ts` files) and sharing them between projects, but if you are publishing or consuming `.d.ts` files, these pitfalls likely apply to you, because `tsc --declaration` transforms `.ts` files into `.d.ts` files.
+
+1. For the reasons laid out in the [`isolatedModules` documentation](/tsconfig#references-to-const-enum-members), that mode is fundamentally incompatible with ambient const enums.
+   This means if you publish ambient const enums, downstream consumers will not be able to use [`isolatedModules`](/tsconfig#isolatedModules) and those enum values at the same time.
+2. You can easily inline values from version A of a dependency at compile time, and import version B at runtime.
+   Version A and B's enums can have different values, if you are not very careful, resulting in [surprising bugs](https://github.com/microsoft/TypeScript/issues/5219#issue-110947903), like taking the wrong branches of `if` statements.
+   These bugs are especially pernicious because it is common to run automated tests at roughly the same time as projects are built, with the same dependency versions, which misses these bugs completely.
+3. [`importsNotUsedAsValues: "preserve"`](/tsconfig#importsNotUsedAsValues) will not elide imports for const enums used as values, but ambient const enums do not guarantee that runtime `.js` files exist.
+   The unresolvable imports cause errors at runtime.
+   The usual way to unambiguously elide imports, [type-only imports](/docs/handbook/modules.html#importing-types), [does not allow const enum values](https://github.com/microsoft/TypeScript/issues/40344), currently.
+
+Here are two approaches to avoiding these pitfalls:
+
+A. Do not use const enums at all.
+   You can easily [ban const enums](https://github.com/typescript-eslint/typescript-eslint/blob/master/docs/getting-started/linting/FAQ.md#how-can-i-ban-specific-language-feature) with the help of a linter.
+   Obviously this avoids any issues with const enums, but prevents your project from inlining its own enums.
+   Unlike inlining enums from other projects, inlining a project's own enums is not problematic and has performance implications.
+B. Do not publish ambient const enums, by deconstifying them with the help of [`preserveConstEnums`](/tsconfig#preserveConstEnums).
+   This is the approach taken internally by the [TypeScript project itself](https://github.com/microsoft/TypeScript/pull/5422).
+   [`preserveConstEnums`](/tsconfig#preserveConstEnums) emits the same JavaScript for const enums as plain enums.
+   You can then safely strip the `const` modifier from `.d.ts` files [in a build step](https://github.com/microsoft/TypeScript/blob/1a981d1df1810c868a66b3828497f049a944951c/Gulpfile.js#L144).
+
+   This way downstream consumers will not inline enums from your project, avoiding the pitfalls above, but a project can still inline its own enums, unlike banning const enums entirely.
+
 ## Ambient enums
 
 Ambient enums are used to describe the shape of already existing enum types.