docs: improve manual around typescript (#8139)

Fixes #9054

docs/typescript/types.md

@@ -0,0 +1,168 @@
+## Types and Type Declarations
+
+One of the design principles of Deno is no _magical_ resolution. When TypeScript
+is type checking a file, it only cares about the types for the file, and the
+`tsc` compiler has a lot of logic to try to resolve those types. By default, it
+expects _ambiguous_ module specifiers with an extension, and will attempt to
+look for the file under the `.ts` specifier, then `.d.ts`, and finally `.js`
+(plus a whole other set of logic when the module resolution is set to `"node"`).
+Deno deals with explicit specifiers.
+
+This can cause a couple problems though. For example, let's say I want to
+consume a TypeScript file that has already been transpiled to JavaScript along
+with a type definition file. So I have `mod.js` and `mod.d.ts`. If I try to
+import `mod.js` into Deno, it will only do what I ask it to do, and import
+`mod.js`, but that means my code won't be as well type checked as if TypeScript
+was considering the `mod.d.ts` file in place of the `mod.js` file.
+
+In order to support this in Deno, Deno has two solutions, of which there is a
+variation of a solution to enhance support. The two main situations you come
+across would be:
+
+- As the importer of a JavaScript module, I know what types should be applied to
+  the module.
+- As the supplier of the JavaScript module, I know what types should be applied
+  to the module.
+
+The latter case is the better case, meaning you as the provider or host of the
+module, everyone can consume it without having to figure out how to resolve the
+types for the JavaScript module, but when consuming modules that you may not
+have direct control over, the ability to do the former is also required.
+
+### Providing types when importing
+
+If you are consuming a JavaScript module and you have either created types (a
+`.d.ts` file) or have otherwise obtained the types, you want to use, you can
+instruct Deno to use that file when type checking instead of the JavaScript file
+using the `@deno-types` compiler hint. `@deno-types` needs to be a single line
+double slash comment, where when used impacts the next import or re-export
+statement.
+
+For example if I have a JavaScript modules `coolLib.js` and I had a separate
+`coolLib.d.ts` file that I wanted to use, I would import it like this:
+
+```ts
+// @deno-types="./coolLib.d.ts"
+import * as coolLib from "./coolLib.js";
+```
+
+When type checking `coolLib` and your usage of it in the file, the
+`coolLib.d.ts` types will be used instead of looking at the JavaScript file.
+
+The pattern matching for the compiler hint is somewhat forgiving and will accept
+quoted and non-question values for the specifier as well as it accepts
+whitespace before and after the equals sign.
+
+### Providing types when hosting
+
+If you are in control of the source code of the module, or you are in control of
+how the file is hosted on a web server, there are two ways to inform Deno of the
+types for a given module, without requiring the importer to do anything special.
+
+#### Using the triple-slash reference directive
+
+Deno supports using the triple-slash reference directive, which adopts the
+reference comment used by TypeScript in TypeScript files to _include_ other
+files and applies it to JavaScript files.
+
+For example, if I had create `coolLib.js` and along side of it I had created my
+type definitions for my library in `coolLib.d.ts` I could do the following in
+the `coolLib.js` file:
+
+```js
+/// <reference path="./coolLib.d.ts" />
+
+// ... the rest of the JavaScript ...
+```
+
+When Deno encounters this directive, it would resolve the `./coolLib.d.ts` file
+and use that instead of the JavaScript file when TypeScript was type checking
+the file, but still load the JavaScript file when running the program.
+
+#### Using X-TypeScript-Types header
+
+Similar to the triple-slash directive, Deno supports a header for remote modules
+that instructs Deno where to locate the types for a given module. For example, a
+response for `https://example.com/coolLib.js` might look something like this:
+
+```
+HTTP/1.1 200 OK
+Content-Type: application/javascript; charset=UTF-8
+Content-Length: 648
+X-TypeScript-Types: ./coolLib.d.ts
+```
+
+When seeing this header, Deno would attempt to retrieve
+`https://example.com/coolLib.d.ts` and use that when type checking the original
+module.
+
+### Important points
+
+#### Type declaration semantics
+
+Type declaration files (`.d.ts` files) follow the same semantics as other files
+in Deno. This means that declaration files are assumed to be module declarations
+(_UMD declarations_) and not ambient/global declarations. It is unpredictable
+how Deno will handle ambient/global declarations.
+
+In addition, if a type declaration imports something else, like another `.d.ts`
+file, its resolution follow the normal import rules of Deno. For a lot of the
+`.d.ts` files that are generated and available on the web, they may not be
+compatible with Deno.
+
+To overcome this problem, some solution providers, like the
+[Skypack CDN](https://www.skypack.dev/), will automatically bundle type
+declarations just like they provide bundles of JavaScript as ESM.
+
+#### Deno Friendly CDNs
+
+There are CDNs which host JavaScript modules that integrate well with Deno.
+
+- [Skypack.dev](https://docs.skypack.dev/skypack-cdn/code/deno) is a CDN which
+  provides type declarations (via the `X-TypeScript-Types` header) when you
+  append `?dts` as a query string to your remote module import statements. For
+  example:
+
+  ```ts
+  import React from "https://cdn.skypack.dev/react?dts";
+  ```
+
+### Behavior of JavaScript when type checking
+
+If you import JavaScript into TypeScript in Deno and there are no types, even if
+you have `checkJs` set to `false` (the default for Deno), the TypeScript
+compiler will still access the JavaScript module and attempt to do some static
+analysis on it, to at least try to determine the shape of the exports of that
+module to validate the import in the TypeScript file.
+
+This is usually never a problem when trying to import a "regular" ES module, but
+in some cases if the module has special packaging, or is a global _UMD_ module,
+TypeScript's analysis of the module can fail and cause misleading errors. The
+best thing to do in this situation is provide some form of types using one of
+the methods mention above.
+
+#### Internals
+
+While it isn't required to understand how Deno works internally to be able to
+leverage TypeScript with Deno well, it can help to understand how it works.
+
+Before any code is executed or compiled, Deno generates a module graph by
+parsing the root module, and then detecting all of its dependencies, and then
+retrieving and parsing those modules, recursively, until all the dependencies
+are retrieved.
+
+For each dependency, there are two potential "slots" that are used. There is the
+code slot and the type slot. As the module graph is filled out, if the module is
+something that is or can be emitted to JavaScript, it fills the code slot, and
+type only dependencies, like `.d.ts` files fill the type slot.
+
+When the module graph is built, and there is a need to type check the graph,
+Deno starts up the TypeScript compiler and feeds it the names of the modules
+that need to be potentially emitted as JavaScript. During that process, the
+TypeScript compiler will request additional modules, and Deno will look at the
+slots for the dependency, offering it the type slot if it is filled before
+offering it the code slot.
+
+This means when you import a `.d.ts` module, or you use one of the solutions
+above to provide alternative type modules for JavaScript code, that is what is
+provided to TypeScript instead when resolving the module.