adds typings for the 'memdown' package (#28832)

types/memdown/index.d.ts

@@ -0,0 +1,20 @@
+// Type definitions for memdown 3.0
+// Project: https://github.com/Level/memdown
+// Definitions by: Meirion Hughes <https://github.com/MeirionHughes>
+//                 Daniel Byrne <https://github.com/danwbyrne>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.3
+
+import { AbstractLevelDOWN } from 'abstract-leveldown';
+
+export interface MemDown<K, V> extends AbstractLevelDOWN<K, V> {}
+
+export interface MemDownConstructor {
+  // tslint:disable-next-line no-unnecessary-generics
+  new <K = any, V = any>(): MemDown<K, V>;
+  // tslint:disable-next-line no-unnecessary-generics
+  <K = any, V = any>(): MemDown<K, V>;
+}
+
+export const MemDown: MemDownConstructor;
+export default MemDown;

types/memdown/memdown-tests.ts

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+import MemDown from 'memdown';
+
+const test = (db: MemDown<string, string>) => {
+  db.open(() => {
+    db.put("key", "value", (err?: Error) => { });
+    db.get("key", (err?: Error) => { });
+    db.get("key", (err: Error | undefined, value: string) => { });
+    db.close(() => {});
+  });
+};
+
+// doesn't need `new` and can be called as a function
+test(new MemDown<string, string>());
+test(new MemDown());
+test(MemDown<string, string>());
+test(MemDown());

types/memdown/tsconfig.json

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+{
+    "compilerOptions": {
+        "module": "commonjs",
+        "lib": [
+            "es6"
+        ],
+        "noImplicitAny": true,
+        "noImplicitThis": true,
+        "strictNullChecks": true,
+        "strictFunctionTypes": true,
+        "baseUrl": "../",
+        "typeRoots": [
+            "../"
+        ],
+        "types": [],
+        "noEmit": true,
+        "forceConsistentCasingInFileNames": true
+    },
+    "files": [
+        "index.d.ts",
+        "memdown-tests.ts"
+    ]
+}

types/memdown/tslint.json

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+{
+  "extends": "dtslint/dt.json" 
+}