Add type definitions

index.d.ts

@@ -0,0 +1,12 @@
+export type TraphConfig<I,O> = {
+  [K in keyof O]: (i: I, o: O) => O[K]
+}
+
+interface TraphTransformer<I,O>  {
+  (input: I): O
+  lazy: (input: I) => O
+}
+
+declare function traph<I = Record<string,any>,O = Record<string,any>>(config: TraphConfig<I,O>): TraphTransformer<I,O>
+
+export default traph

index.test-d.ts

@@ -0,0 +1,23 @@
+import { expectType, expectAssignable, expectError } from "tsd"
+import traph from "."
+
+type Input = { in: number[] }
+type Output = { out: number }
+
+const stats = traph<Input, Output>({
+  out: (i,o) => {
+    expectType<Input>(i)
+    expectType<Output>(o)
+    return i.in.length
+  }
+})
+
+expectAssignable<(i: Input) => Output>(stats)
+expectType<(i: Input) => Output>(stats.lazy)
+
+declare const data: Input
+expectType<Output>(stats(data))
+
+expectError(traph<Input,Output>({}))
+expectError(traph<Input,Output>({out: "string value"}))
+expectError(traph<Input,Output>({out: (i,o) => "string value"}))

package.json

@@ -10,12 +10,15 @@
     "lint": "eslint index.js",
     "test:watch": "ava --watch",
     "test": "npm run lint && ava",
-    "prepublish": "npm run build && npm test"
+    "test:types": "tsd",
+    "prepublish": "npm run build && npm test && npm run test:types"
   },
   "main": "dist/index.js",
   "files": [
-    "dist"
+    "dist",
+    "index.d.ts"
   ],
+  "type": "index.d.ts",
   "ava": {
     "require": [
       "babel-register"
@@ -31,7 +34,8 @@
     "eslint-config-standard": "7.0.0-beta.0",
     "eslint-plugin-ava": "^4.1.0",
     "eslint-plugin-promise": "^3.4.1",
-    "eslint-plugin-standard": "^2.0.1"
+    "eslint-plugin-standard": "^2.0.1",
+    "tsd": "^0.11.0"
   },
   "dependencies": {}
 }