Updates entry_point to remove support for returning a string from an entry point.

Refs #25.

Since the original renderer tool was removed, it is no longer ever valid for users to return a `string` or `Promise<string>` type, meaning there is no need to support it. This removes that case from `entry_point` and removes an assertion error from `renderer`.

Author: dgp1130

packages/renderer/BUILD.bazel

@@ -19,7 +19,6 @@ ts_library(
         ":entry_point",
         "//common:binary",
         "//common:formatters",
-        "//common/models:prerender_resource",
         "@npm//@types/node",
         "@npm//@types/yargs",
     ],

packages/renderer/entry_point.ts

@@ -6,13 +6,18 @@ import { PrerenderResource } from 'rules_prerender/common/models/prerender_resou
  * the default function export, and validates the result before returning it.
  * 
  * @param entryPoint A path to a JavaScript CommonJS module. This module should
- *     have a default export of a function. This function accepts no arguments
- *     and returns a `string` or a `Promise<string>`. The module is imported
- *     into the NodeJS runtime and invoked, propagating the return value to the
- *     caller.
+ *     have a default export of a function with the type:
+ * 
+ *     ```
+ *     () => Iterable<PrerenderResource> | Promise<Iterable<PrerenderResource>>
+ *         | AsyncIterable<PrerenderResource>
+ *     ```
+ * 
+ *     The module is imported into the NodeJS runtime and invoked, propagating
+ *     the return value to the caller.
+ * @returns The returned value of the invoked entry point.
  */
 export async function invoke(entryPoint: string): Promise<
-    string
     | Iterable<PrerenderResource>
     | AsyncIterable<PrerenderResource>
 > {
@@ -33,7 +38,6 @@ export async function invoke(entryPoint: string): Promise<
     }
 
     const rendered = await defaultExport() as unknown;
-    if (typeof rendered === 'string') return rendered;
     if (isIterable(rendered)) {
         return rendered as Iterable<PrerenderResource>;
     }
@@ -43,9 +47,6 @@ export async function invoke(entryPoint: string): Promise<
 
     throw new Error(`Entry point (${entryPoint}) provided a default export`
         + ` which returned a value that is not one of:\n${[
-            'string',
-            'Promise<string>',
-            'Promise<PrerenderResource>',
             'Iterable<PrerenderResource>',
             'Promise<Iterable<PrerenderResource>>',
             'AsyncIterable<PrerenderResource>',

packages/renderer/entry_point_test.ts

@@ -6,33 +6,6 @@ import * as importLib from 'rules_prerender/packages/renderer/dynamic_import';
 
 describe('entry_point', () => {
     describe('invoke()', () => {
-        it('invokes the given entry point and returns its string value', async () => {
-            const defaultExport = jasmine.createSpy('defaultExport')
-                    .and.returnValue('Hello, World!');
-            spyOn(importLib, 'dynamicImport').and.resolveTo({
-                default: defaultExport,
-            });
-
-            const rendered = await invoke('foo.js');
-
-            expect(importLib.dynamicImport).toHaveBeenCalledOnceWith('foo.js');
-            expect(defaultExport).toHaveBeenCalledOnceWith();
-
-            expect(rendered).toBe('Hello, World!');
-        });
-
-        it('invokes the given entry point and awaits its `Promise<string>` value', async () => {
-            const defaultExport = jasmine.createSpy('defaultExport')
-                    .and.resolveTo('Hello, World!');
-            spyOn(importLib, 'dynamicImport').and.resolveTo({
-                default: defaultExport,
-            });
-
-            const rendered = await invoke('foo.js');
-
-            expect(rendered).toBe('Hello, World!');
-        });
-
         it('invokes the given entry point and returns its `Iterable<PrerenderResource>` value', async () => {
             const defaultExport =
                 jasmine.createSpy('defaultExport').and.returnValue([

packages/renderer/renderer.ts

@@ -3,7 +3,6 @@ import * as path from 'path';
 import * as yargs from 'yargs';
 import { main } from 'rules_prerender/common/binary';
 import { mdSpacing } from 'rules_prerender/common/formatters';
-import { PrerenderResource } from 'rules_prerender/common/models/prerender_resource';
 import { invoke } from 'rules_prerender/packages/renderer/entry_point';
 
 main(async () => {
@@ -34,27 +33,14 @@ main(async () => {
         .argv;
 
     // Invoke the provided entry point.
-    let resources: string | Iterable<PrerenderResource>
-        | AsyncIterable<PrerenderResource>;
+    let resources: PromiseValue<ReturnType<typeof invoke>>;
     try {
         resources = await invoke(entryPoint);
     } catch (err) {
         console.error(err.message);
         return 1;
     }
 
-    // Validate the result of the entry point.
-    if (typeof resources === 'string') {
-        console.error(
-            `Expected entry point (${entryPoint}) to return one of:\n${[
-                'Iterable<PrerenderResource>',
-                'Promise<Iterable<PrerenderResource>>',
-                'AsyncIterable<PrerenderResource>',
-            ].join('\n')}\n\nInstead, got:\n${stringify(resources)}`,
-        );
-        return 1;
-    }
-
     // Write each resource to its file.
     const writes = [] as Promise<void>[];
     for await (const resource of resources) {
@@ -75,11 +61,6 @@ main(async () => {
     return 0;
 });
 
-function stringify(value: unknown): string {
-    if (typeof value === 'string') return value;
-    if (typeof value === 'object') {
-        const obj = value as Record<string, unknown>;
-        if (obj.toString) return obj.toString();
-    }
-    return JSON.stringify(value);
-}
+// Extracts the generic value type from an input `Promise`.
+type PromiseValue<T extends Promise<unknown>> =
+    T extends Promise<infer V> ? V : never;

packages/renderer/renderer_test.ts

@@ -162,32 +162,4 @@ module.exports = 'Hello, World!'; // Not a function...
         });
         expect(outputFiles).toEqual([]);
     });
-
-    it('fails from user code returning a bad type', async () => {
-        await fs.mkdir(`${tmpDir.get()}/output`);
-        await fs.writeFile(`${tmpDir.get()}/foo.js`, `
-module.exports = () => {
-    return 'Hello, World!'; // Not an iterable.
-};
-        `.trim());
-
-        const { code, stdout, stderr } = await run({
-            entryPoint: `${tmpDir.get()}/foo.js`,
-            outputDir: `${tmpDir.get()}/output`,
-        });
-
-        expect(code).toBe(1);
-        expect(stdout).toBe('');
-        expect(stderr).toContain(`${tmpDir.get()}/foo.js`);
-        expect(stderr).toContain('return one of:');
-        expect(stderr).not.toContain('\\n'); // String should not be escaped.
-        // Stack trace should not be displayed for user-fault errors.
-        expect(stderr).not.toContain('    at ');
-        
-        // No output files should be written.
-        const outputFiles = await fs.readdir(`${tmpDir.get()}/output`, {
-            withFileTypes: true,
-        });
-        expect(outputFiles).toEqual([]);
-    });
 });