Update FAQ.md

Author: Raynos

FAQ.md

@@ -39,3 +39,58 @@ class App {
   }
 }
 ```
+
+## I want to share some helper interfaces or aliases globally in my JSDoc project.
+
+Turns out that you can share interfaces / type aliases globally in a project without copy pasting
+or adding lots of `@typedef {import(...)} ...` statements.
+
+The most important thing to do is to `declare`. 
+
+```ts
+// typedefs.d.ts
+declare type Result<T> = {
+  err: Error,
+  data?: undefined
+} | {
+  err?: undefined,
+  data: T
+}
+```
+
+Then in any javscript file you can use `Result<null>` or `Result<string[]>` etc.
+
+Do make sure that this file is included in your `jsconfig.json`
+
+```json
+{
+  "compilerOptions": { ... },
+  "include": [
+    "src/**/*.js",
+    "src/**/*.d.ts"
+  ]
+}
+```
+
+I've set up my project to just include all `.d.ts`
+
+### Can I use `import` with this `typedefs.d.ts`
+
+Yes you can, however adding [`import` or `export` turns it into a module](https://stackoverflow.com/a/57040462/419970) and its no longer global.
+
+You will have to write `declare global { ... }` to be able to declare global interfaces/type aliases from a module.
+
+```
+// typedefs.d.ts
+import * as events from 'events'
+
+declare global {
+  type Result<T> = {
+    err: Error,
+    data?: undefined
+  } | {
+    err?: undefined,
+    data: T
+  }
+}
+```