Bower 1.8.x: Main field precedence compatibility boundary remains undefined

Direct answer

When both bower.json and package.json exist in a package, Bower 1.8.x uses the main field from bower.json as the authoritative entry point. The package.json main field serves only as a fallback when bower.json is absent or omits the main property.

How the precedence works

Bower's manifest resolution logic treats bower.json as the primary configuration source for all settings, including main, dependencies, and ignore. During bower install, the resolver reads bower.json first. If that file exists and contains a main array or string, those paths define the files mapped into the consumer's bower_components directory. Only if bower.json is missing entirely, or if it lacks a main field, does Bower fall back to reading package.json.

Documentation status

The official Bower documentation describes main as a "hint for file mapping" but does not explicitly state the precedence order between the two manifest formats. The behavior described above is derived from the implementation in the 1.8.x source (specifically lib/core/resolver.js and lib/util/readJson.js) and has been observed consistently across the 1.8.0–1.8.14 releases. Because Bower is deprecated and unmaintained, this implementation-defined behavior will not receive further clarification or guarantees.

Verification steps

1. Create a test package with both manifests:

   mkdir test-pkg && cd test-pkg
   cat > bower.json <<'EOF'
   {"name": "test-pkg", "main": "bower-entry.js"}
   EOF
   cat > package.json <<'EOF'
   {"name": "test-pkg", "main": "npm-entry.js"}
   EOF
   echo "console.log('from bower')" > bower-entry.js
   echo "console.log('from npm')" > npm-entry.js
   

2. In a separate consumer directory, run bower install ../test-pkg (or publish to a local registry).
3. Inspect bower_components/test-pkg/ — the mapped entry file will be bower-entry.js.
4. Remove bower.json and repeat; the resolved entry switches to npm-entry.js.

Caveats

• Custom .bowerrc settings (e.g., directory, cwd) can alter installation paths but do not change the manifest precedence order.
• If bower.json exists but its main is an empty array or invalid, Bower may fall back to package.json — this edge case is not formally specified.
• Bower has been deprecated since 2017; consider migrating to npm/yarn/pnpm with a bundler for long-term maintenance.