fix: added coverage in tests and updated docs

Author: A.P.A. Slaa

README.md

@@ -13,7 +13,7 @@ No dependencies, focus on runtime safety.
 Key features:
 - Load `.env` file (ignores comments, blank lines, quotes) and merge into `process.env`.
 - Access variables via `.string()`, `.number()`, `.boolean()` with defaults.
-- Check keys with `.has()`, `.defined()`.
+- Check keys with `.has()`, `.defined()`, `.assert()`.
 - Get `env.dev` boolean for “development vs production” mode.
 - Collect variables with a common prefix via `.collection()`, optional prefix removal & reviver.
 - Utility method `.utils.select()` for feature‐flag style branching.
@@ -84,7 +84,7 @@ When you have many environment variables prefixed in a group:
 
 ```ts
 // .env
-API_URL=https://api.example.com
+API_URL="https://api.example.com"
 API_KEY=abcdef
 API_TIMEOUT=5000
 
@@ -120,11 +120,12 @@ const mode = env.utils.select("FEATURE_X", "enabled", "disabled");
 ## API Reference
 
 | Method                                           | Description                                                                                         |
-| ------------------------------------------------ | --------------------------------------------------------------------------------------------------- |
+|--------------------------------------------------|-----------------------------------------------------------------------------------------------------|
 | `env.string(key, default?)`                      | Return the variable as a string (or default).                                                       |
 | `env.number(key, default?)`                      | Parse variable to number (or default).                                                              |
 | `env.boolean(key, default?)`                     | Parse variable to boolean (or default).                                                             |
 | `env.has(key)`                                   | Returns true if key exists in `process.env`.                                                        |
+| `env.assert([keys], error_builder?)`             | Throws an error if one or more keys doesn't exists in `process.env`.                                |
 | `env.defined(key)`                               | Returns true if key exists and value is not `undefined`.                                            |
 | `env.dev`                                        | Boolean flag: true if `NODE_ENV !== "production"`.                                                  |
 | `env.collection(prefix, options?)`               | Get an object of all env keys starting with `prefix`. Options include `removePrefix` and `reviver`. |

src/index.ts

@@ -67,7 +67,7 @@ export const env = Object.freeze({
     },
     assert(keys: Uppercase<string>[], error_builder: (missing_keys: string[]) => string | Error = ((missing_keys) => new Error(`Missing required keys(${missing_keys.join()}) in environment`)),) {
         const missing_keys: string[] = [];
-        keys.forEach((key) => this.has(key) ? missing_keys.push(key) : void 0);
+        keys.forEach((key) => this.has(key) ? void 0: missing_keys.push(key));
         if (missing_keys.length > 0) {
             const result = error_builder(missing_keys);
             if (typeof result === "string") throw new Error(result);

tests/env.test.ts

@@ -122,4 +122,52 @@ describe("env library", () => {
         env = (await import("../src")).env;
         expect(env.raw).toBeTypeOf('object');
     });
+
+    it("assert throws error when required keys are missing (default error)", async () => {
+        mockExistsSync.mockReturnValue(false);
+        env = (await import("../src")).env;
+
+        // Define one key
+        env.string("EXISTING_KEY", "present");
+
+        // Assert on a missing key
+        expect(() => env.assert(["MISSING_KEY"])).toThrow(
+            'Missing required keys(MISSING_KEY) in environment'
+        );
+    });
+
+    it("assert throws custom error when required keys are missing", async () => {
+        mockExistsSync.mockReturnValue(false);
+        env = (await import("../src")).env;
+
+        const customErrorBuilder = (missing: string[]) =>
+            new Error(`Custom: ${missing.join(", ")} are missing!`);
+
+        expect(() => env.assert(["A", "B"], customErrorBuilder)).toThrow(
+            "Custom: A, B are missing!"
+        );
+    });
+
+    it("assert throws custom string error (converted to Error)", async () => {
+        mockExistsSync.mockReturnValue(false);
+        env = (await import("../src")).env;
+
+        const stringErrorBuilder = (missing: string[]) =>
+            `STRING ERROR: missing ${missing.join("+")}`;
+
+        expect(() => env.assert(["X"], stringErrorBuilder)).toThrow(
+            "STRING ERROR: missing X"
+        );
+    });
+
+    it("assert does nothing when all required keys exist", async () => {
+        mockExistsSync.mockReturnValue(false);
+        env = (await import("../src")).env;
+
+        env.string("KEY1", "val1");
+        env.string("KEY2", "val2");
+
+        // Should not throw
+        expect(() => env.assert(["KEY1", "KEY2"])).not.toThrow();
+    });
 });