Conditionally append NPM_TOKEN to .npmrc (#545)

* fix: conditionally append NPM_TOKEN to .npmrc for trusted publishing support

The .npmrc generation now intelligently handles both traditional NPM token
authentication and trusted publishing scenarios:

- Only appends auth token to .npmrc when NPM_TOKEN is defined (not undefined)
- Uses strict comparison (!== undefined) instead of truthy checks
- Provides informative logging when no NPM_TOKEN is present
- Maintains full backward compatibility with existing workflows

This fixes issues with trusted publishing where OIDC tokens from GitHub
Actions are used instead of NPM_TOKEN, preventing 'undefined' from being
written to the registry auth configuration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* Update src/index.ts

Co-authored-by: Mateusz Burzyński <[email protected]>

* simplify

* tweak changeset

---------

Co-authored-by: Claude <[email protected]>
Co-authored-by: Mateusz Burzyński <[email protected]>

Author: 3 people

.changeset/npmrc-trusted-publishing.md

@@ -0,0 +1,5 @@
+---
+"@changesets/action": patch
+---
+
+The `.npmrc` generation now intelligently handles both traditional NPM token authentication and trusted publishing scenarios by only appending the auth token when `NPM_TOKEN` is defined. This prevents 'undefined' from being written to the registry configuration when using OIDC tokens from GitHub Actions trusted publishing.

src/index.ts

@@ -67,32 +67,41 @@ const getOptionalInput = (name: string) => core.getInput(name) || undefined;
         "No changesets found. Attempting to publish any unpublished packages to npm"
       );
 
-      let userNpmrcPath = `${process.env.HOME}/.npmrc`;
-      if (await fileExists(userNpmrcPath)) {
-        core.info("Found existing user .npmrc file");
-        const userNpmrcContent = await fs.readFile(userNpmrcPath, "utf8");
-        const authLine = userNpmrcContent.split("\n").find((line) => {
-          // check based on https://github.com/npm/cli/blob/8f8f71e4dd5ee66b3b17888faad5a7bf6c657eed/test/lib/adduser.js#L103-L105
-          return /^\s*\/\/registry\.npmjs\.org\/:[_-]authToken=/i.test(line);
-        });
-        if (authLine) {
-          core.info(
-            "Found existing auth token for the npm registry in the user .npmrc file"
-          );
+      if (process.env.NPM_TOKEN) {
+        const userNpmrcPath = `${process.env.HOME}/.npmrc`;
+
+        if (await fileExists(userNpmrcPath)) {
+          core.info("Found existing user .npmrc file");
+          const userNpmrcContent = await fs.readFile(userNpmrcPath, "utf8");
+          const authLine = userNpmrcContent.split("\n").find((line) => {
+            // check based on https://github.com/npm/cli/blob/8f8f71e4dd5ee66b3b17888faad5a7bf6c657eed/test/lib/adduser.js#L103-L105
+            return /^\s*\/\/registry\.npmjs\.org\/:[_-]authToken=/i.test(line);
+          });
+          if (authLine) {
+            core.info(
+              "Found existing auth token for the npm registry in the user .npmrc file"
+            );
+          } else {
+            core.info(
+              "Didn't find existing auth token for the npm registry in the user .npmrc file, creating one"
+            );
+            await fs.appendFile(
+              userNpmrcPath,
+              `\n//registry.npmjs.org/:_authToken=${process.env.NPM_TOKEN}\n`
+            );
+          }
         } else {
           core.info(
-            "Didn't find existing auth token for the npm registry in the user .npmrc file, creating one"
+            "No user .npmrc file found, creating one with NPM_TOKEN used as auth token"
           );
-          await fs.appendFile(
+          await fs.writeFile(
             userNpmrcPath,
-            `\n//registry.npmjs.org/:_authToken=${process.env.NPM_TOKEN}\n`
+            `//registry.npmjs.org/:_authToken=${process.env.NPM_TOKEN}\n`
           );
         }
       } else {
-        core.info("No user .npmrc file found, creating one");
-        await fs.writeFile(
-          userNpmrcPath,
-          `//registry.npmjs.org/:_authToken=${process.env.NPM_TOKEN}\n`
+        core.info(
+          "No NPM_TOKEN found - assuming trusted publishing or npm is already authenticated"
         );
       }