Exploit generics to assert typings of custom data

types/vfile/index.d.ts

@@ -24,7 +24,8 @@ declare namespace vfile {
         position?: Position;
     }
 
-    interface VFileParams {
+    interface VFileParamsBase<D> {
+        data?: D;
         contents?: Contents;
         path?: string;
         basename?: string;
@@ -34,6 +35,8 @@ declare namespace vfile {
         cwd?: string;
     }
 
+    type VFileParams<C, D> = VFileParamsBase<D> & C;
+
     interface VFileError {
         ruleId: string | null;
         name: string;
@@ -56,13 +59,13 @@ declare namespace vfile {
 
     type ToString = (encoding?: BufferEncoding) => string;
 
-    interface VFile {
-        (input?: Contents | VFile | VFileParams): VFile;
+    interface VFileBase<C, D> {
+        <C>(input?: Contents | VFile<C, D> | VFileParams<C, D>): VFile<C, D>;
         message: Message;
         fail: Fail;
         info: Info;
         history: string[];
-        data: {};
+        data: D;
         messages: VFileError[];
         contents: Contents;
         path?: string;
@@ -73,8 +76,10 @@ declare namespace vfile {
         cwd: string;
         toString: ToString;
     }
+
+    type VFile<C, D> = VFileBase<C, D> & C;
 }
 
-declare const vfile: vfile.VFile;
+declare const vfile: vfile.VFile<{}, {}>;
 
 export = vfile;

types/vfile/vfile-tests.ts

@@ -1,6 +1,13 @@
 import vfile = require("vfile");
 
-const file = vfile({path: '~/example.txt', contents: 'Alpha *braavo* charlie.'});
+const file = vfile({
+    path: '~/example.txt',
+    contents: 'Alpha *braavo* charlie.',
+    custom: 'Custom tango',
+    data: {
+        custom: 12345
+    }
+});
 
 file.path; // => '~/example.txt'
 file.dirname; // => '~'
@@ -16,3 +23,7 @@ file.history; // => ['~/example.txt', '~/example.md', '~/index.text']
 file.message('`braavo` is misspelt; did you mean `bravo`?', {line: 1, column: 8});
 
 console.log(file.messages);
+
+// Keep typings of custom data
+const custom: string = file.custom; // 'Custom tango'
+const dataCustom: number = file.data.custom; // 12345