core:deep pick

packages/core/package.json

@@ -51,11 +51,12 @@
   "dependencies": {
     "tslog": "^3.3.3",
     "tsup": "^8.3.5",
+    "type-fest": "^4.34.1",
     "zod": "^3.24.1"
   },
   "devDependencies": {
-    "@types/node": "^22.12.0",
     "@repo/typescript-config": "workspace:*",
+    "@types/node": "^22.12.0",
     "typescript": "^5.7.3"
   },
   "scripts": {

packages/core/src/pick.ts

@@ -0,0 +1,78 @@
+import { Split } from 'type-fest'
+
+/**
+ * Recursively pick the sub-object from T according to the array of keys in Keys.
+ *
+ * If Keys = ['address', 'coords', 'lat'], 
+ * we want to produce { address: { coords: { lat: number } } } at the type level.
+ */
+type DeepPickArray<T, Keys extends readonly string[]> =
+    Keys extends []
+    ? T
+    : Keys extends [infer Head, ...infer Tail]
+    ? Head extends keyof T
+    ? {
+        [P in Head]: Tail extends string[]
+        ? DeepPickArray<T[Head], Tail>
+        : never
+    }
+    : never
+    : never;
+
+/**
+ * Given a single path string (e.g. "address/coords/lat") and a separator (default = "."),
+ * parse that path into an array via `Split`, then recursively pick the resulting type.
+ */
+type DeepPickOne<
+    T,
+    Path extends string,
+    Sep extends string = "."
+> = DeepPickArray<T, Split<Path, Sep>>;
+
+/**
+ * If DeepPickOne<T, Path, Sep> yields `never` (meaning invalid path),
+ * we allow a fallback type D (default = undefined).
+ */
+type DeepPickValue<
+    T,
+    Path extends string,
+    Sep extends string = "/",
+    D = undefined
+> = DeepPickOne<T, Path, Sep> extends infer R
+    ? [R] extends [never]
+    ? D
+    : R
+    : D;
+
+export function pick <
+    T,
+    Path extends string,
+    Sep extends string = "/",
+    D = undefined
+>(
+    obj: T,
+    path: Path,
+    options?: {
+        separator?: Sep;
+        defaultValue?: D;
+    }
+): DeepPickValue<T, Path, Sep, D> {
+    // Determine separator (default to "/")
+    const separator = options?.separator ?? "/";
+    // Split the path into keys
+    const keys = path.split(separator);
+
+    // Traverse the object
+    let current: any = obj;
+    for (const key of keys) {
+        if (current == null || !(key in current)) {
+            // If we can't go further, return defaultValue (if provided)
+            return (options?.defaultValue as any)
+                ?? (undefined as DeepPickValue<T, Path, Sep, D>);
+        }
+        current = current[key];
+    }
+
+    // If we successfully traversed the entire path, return the value
+    return current as DeepPickValue<T, Path, Sep, D>;
+}