Fix node-version-file interprets entire package.json as a version (#865)

src/main.ts

@@ -105,7 +105,17 @@ function resolveVersionInput(): string {
       );
     }
 
-    version = parseNodeVersionFile(fs.readFileSync(versionFilePath, 'utf8'));
+    const parsedVersion = parseNodeVersionFile(
+      fs.readFileSync(versionFilePath, 'utf8')
+    );
+
+    if (parsedVersion) {
+      version = parsedVersion;
+    } else {
+      core.warning(
+        `Could not determine node version from ${versionFilePath}. Falling back`
+      );
+    }
 
     core.info(`Resolved ${versionFileInput} as ${version}`);
   }

src/util.ts

@@ -1,13 +1,31 @@
 import * as core from '@actions/core';
 import * as exec from '@actions/exec';
 
-export function parseNodeVersionFile(contents: string): string {
+export function parseNodeVersionFile(contents: string): string | null {
   let nodeVersion: string | undefined;
 
   // Try parsing the file as an NPM `package.json` file.
   try {
-    nodeVersion = JSON.parse(contents).volta?.node;
-    if (!nodeVersion) nodeVersion = JSON.parse(contents).engines?.node;
+    const manifest = JSON.parse(contents);
+
+    // JSON can parse numbers, but that's handled later
+    if (typeof manifest === 'object') {
+      nodeVersion = manifest.volta?.node;
+      if (!nodeVersion) nodeVersion = manifest.engines?.node;
+
+      // if contents are an object, we parsed JSON
+      // this can happen if node-version-file is a package.json
+      // yet contains no volta.node or engines.node
+      //
+      // if node-version file is _not_ json, control flow
+      // will not have reached these lines.
+      //
+      // And because we've reached here, we know the contents
+      // *are* JSON, so no further string parsing makes sense.
+      if (!nodeVersion) {
+        return null;
+      }
+    }
   } catch {
     core.info('Node version file is not JSON file');
   }