feat: introduce record types

Author: abdala

README.md

@@ -227,6 +227,15 @@ if (is(data, dog)) {
 }
 ```
 
+### Record Types
+
+It's also possible to use `record` passing the type for the key and value:
+
+```ts
+assert(value, record(String, String));
+// value.foo is a string
+```
+
 ### Union Types
 
 The package exports a `union` function to check if a value is of either of the given types:

src/index.ts

@@ -179,3 +179,23 @@ export function assert<T extends Schema>(
     throw new TypeError(`${mismatch[0]} does not match the schema.`);
   }
 }
+
+/**
+ *  Valid Record keys
+ */
+export type RecordKeys =
+  | StringConstructor
+  | NumberConstructor
+  | string
+  | number
+
+/**
+ * Creates a type guard that checks if a value matches a given Record<K, V>.
+ */
+export function record<const K extends RecordKeys, const V extends Schema>(key: K, value: V) {
+  return (v: unknown): v is Record<TypeFromSchema<K>, TypeFromSchema<V>> =>
+    typeof v === 'object' 
+    && v !== null 
+    && Object.values(v).every((y) => is(y, value)) 
+    && Object.keys(v).every((y) => is(y, key))
+}

src/test/index.ts

@@ -5,6 +5,7 @@ import {
   diff,
   is,
   optional,
+  record,
   typeGuard,
   union,
   unknown,
@@ -301,3 +302,11 @@ tap.test("deeply nested schemas", async (t) => {
   const value: S = {};
   t.ok(is(value, schema));
 });
+
+tap.test("record", async (t) => {
+  const obj: unknown = { foo: "foo" };
+  assert(obj, record(String, String));
+  t.throws(() => {
+    assert(obj, record(String, Number));
+  }, TypeError);
+});