fix #4420, close #4418: metafile JSON regression

Author: evanw

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+* Fix a regression when `metafile` is enabled ([#4420](https://github.com/evanw/esbuild/issues/4420), [#4418](https://github.com/evanw/esbuild/pull/4418))
+
+    This release fixes a regression introduced by the previous release. When `metafile: true` was enabled in esbuild's JavaScript API, builds with build errors were incorrectly throwing an error about an empty JSON string instead of an object containing the build errors.
+
 * Allow `es2025` as a target in `tsconfig.json` ([#4432](https://github.com/evanw/esbuild/issues/4432))
 
     TypeScript recently [added `es2025`](https://devblogs.microsoft.com/typescript/announcing-typescript-6-0/#es2025-option-for-target-and-lib) as a compilation target, so esbuild now supports this in the `target` field of `tsconfig.json` files, such as in the following configuration file:

lib/shared/common.ts

@@ -953,7 +953,7 @@ function buildOrContextImpl(
       const originalErrors = result.errors.slice()
       const originalWarnings = result.warnings.slice()
       if (response!.outputFiles) result.outputFiles = response!.outputFiles.map(convertOutputFiles)
-      if (response!.metafile) result.metafile = parseJSON(response!.metafile)
+      if (response!.metafile && response!.metafile.length) result.metafile = parseJSON(response!.metafile)
       if (response!.mangleCache) result.mangleCache = response!.mangleCache
       if (response!.writeToStdout !== void 0) console.log(protocol.decodeUTF8(response!.writeToStdout).replace(/\n$/, ''))
       runOnEndCallbacks(result, (onEndErrors, onEndWarnings) => {

scripts/js-api-tests.js

@@ -1700,6 +1700,35 @@ body {
     assert.deepStrictEqual(json.outputs[fileKey].inputs, { [makePath(file)]: { bytesInOutput: 14 } })
   },
 
+  // See: https://github.com/evanw/esbuild/issues/4420
+  async metafileEmpty({ esbuild, testDir }) {
+    const entry = path.join(testDir, 'entry.ts')
+    const unknown = path.join(testDir, 'a.unknown')
+    await writeFileAsync(entry, `
+      import contents from "./a.unknown";
+      console.log(contents);
+    `)
+    await writeFileAsync(unknown, '')
+
+    // Generate a build with an empty metafile JSON string due to an error
+    let error
+    try {
+      await esbuild.build({
+        entryPoints: [entry],
+        bundle: true,
+        outdir: testDir,
+        metafile: true,
+        logLevel: 'silent',
+      })
+    } catch (e) {
+      error = e
+    }
+
+    // Make sure we get an error message about the unknown file extension
+    assert.strictEqual(error.errors.length, 1)
+    assert(error.errors[0].text.includes('.unknown'))
+  },
+
   // Test in-memory output files
   async writeFalse({ esbuild, testDir }) {
     const input = path.join(testDir, 'in.js')