make promise check a bit DRYer

Author: EskiMojo14

packages/core/src/index.ts

@@ -1,5 +1,5 @@
 import type { StandardSchemaDictionary, StandardSchemaV1 } from "./standard";
-import { parseWithDictionary } from "./standard";
+import { ensureSynchronous, parseWithDictionary } from "./standard";
 
 export type { StandardSchemaV1, StandardSchemaDictionary };
 
@@ -292,12 +292,13 @@ export function createEnv<
         ..._shared,
       };
 
-  const parsed = opts.createFinalSchema?.(finalSchemaShape as never, isServer)["~standard"].validate(runtimeEnv)
-        ?? parseWithDictionary(finalSchemaShape, runtimeEnv)
+  const parsed =
+    opts
+      .createFinalSchema?.(finalSchemaShape as never, isServer)
+      ["~standard"].validate(runtimeEnv) ??
+    parseWithDictionary(finalSchemaShape, runtimeEnv);
 
-  if (parsed instanceof Promise) {
-    throw new Error("Validation must be synchronous");
-  }
+  ensureSynchronous(parsed, "Validation must be synchronous");
 
   const onValidationError =
     opts.onValidationError ??

packages/core/src/standard.ts

@@ -88,21 +88,28 @@ export namespace StandardSchemaDictionary {
   };
 }
 
+export function ensureSynchronous<T>(
+  value: T | Promise<T>,
+  message: string,
+): asserts value is T {
+  if (value instanceof Promise) {
+    throw new Error(message);
+  }
+}
+
 export function parseWithDictionary<TDict extends StandardSchemaDictionary>(
   dictionary: TDict,
   value: Record<string, unknown>,
 ): StandardSchemaV1.Result<StandardSchemaDictionary.InferOutput<TDict>> {
   const result: Record<string, unknown> = {};
   const issues: StandardSchemaV1.Issue[] = [];
   for (const key in dictionary) {
-    const schema = dictionary[key];
-    const prop = value[key];
-    const propResult = schema["~standard"].validate(prop);
-    if (propResult instanceof Promise) {
-      throw new Error(
-        `Validation must be synchronous, but ${key} returned a Promise.`,
-      );
-    }
+    const propResult = dictionary[key]["~standard"].validate(value[key]);
+    ensureSynchronous(
+      propResult,
+      `Validation must be synchronous, but ${key} returned a Promise.`,
+    );
+
     if (propResult.issues) {
       issues.push(
         ...propResult.issues.map((issue) => ({