Pluggable Resolvers

Pluggable resolvers allow you to use resolvers created by 3rd party JavaScript developers — including overriding default resolvers used by Bower.

For example, resolvers can be used for:

Pluggable resolvers were introduced in Bower 1.5. Please make sure your Bower version is correct (bower --version).


A Pluggable Resolver is just an npm package that you install as devDependency in the package.json of your repository, or install globally with npm install -g.

Declare what Pluggable resolvers your project uses by adding entries to the resolvers section of .bowerrc.

  "resolvers": [

Bower tries to use resolvers in the order specified. If no custom resolver matches the source being processed, Bower fallbacks to default resolvers (git, github, filesystem, svn, registry).

You can find the list of available Bower resolvers on npm website.


As mentioned, custom resolvers are npm packages with specific a API described below.

The package.json should not list bower as a dependency or peerDependency (both have undesired behavior in npm 2.x, and we don’t want you to use bower internals). Instead, you can check for proper environment in resolver’s factory by reading provided bower.version parameter and use any other packages on npm (like request).

Packages should list bower-resolver as one of the keywords in package.json. Resolvers should also follow semver specification.

Here is how an example package.json of a custom resolver can look like:

  "name": "custom-bower-resolver",
  "version": "1.0.0",
  "keywords": ["bower-resolver"],
  "main": "index.js",
  "dependencies": {
    "request": "^2.61.0"

The index.js should export factory for resolver, as follows:

var tmp = require('tmp');

 * Factory function for resolver
 * It is called only one time by Bower, to instantiate resolver.
 * You can instantiate here any caches or create helper functions.
module.exports = function resolver (bower) {

  // Resolver factory returns an instance of resolver
  return {

    // Match method tells whether resolver supports given source
    // It can return either boolean or promise of boolean
    match: function (source) {
      return source.indexOf('svn://') === 0

    // Optional:
    // Can resolve or normalize sources, like:
    // "jquery" => "git://github.com/jquery/jquery.git"
    locate: function (source) {
      return source;

    // Optional:
    // Allows to list available versions of given source.
    // Bower chooses matching release and passes it to "fetch"
    releases: function (source) {
      return [
        { target: 'v1.0.0', version: '1.0.0' },
        { target: 'v1.0.1', version: '1.0.1' }

    // It downloads package and extracts it to temporary directory
    // You can use npm's "tmp" package to tmp directories
    // See the "Resolver API" section for details on this method
    fetch: function (endpoint, cached) {
      // If cached version of package exists, re-use it
      if (cached && cached.version) {

      var tempDir = tmp.dirSync();

      // ... download package to tempDir

      return {
        tempPath: tempDir.name,
        removeIgnores: true

If you need something more solid, see this real world example: Mercurial Resolver.

Resolver API

Resolver package

var plugResolver = require('pluggable-resolver')

var resolver = plugResolver({
  version: '1.5.0',
  config: {...},
  logger: logger
  • resolver: Resolver - instance of the resolver.
  • version: String - Bower’s version that instantiates resolver. You can validate it.
  • config: Object - Bower’s config. You can ask authors to put extra configuration in it.
  • logger: Object - Bower’s logger. Use it to output important warnings / information.

plugResolver() returns an instance of the resolver with the API described below.



Tells Bower whether to use or not use this resolver for some source.

var isMatched = resolver.match( source )
  • source: String - source from bower.json, like git://github.com/jquery/jquery.git
  • isMatched: Boolean - Returns a boolean that tells whether resolver can handle given source (either by locating them with locate method, or fetching it with fetch + optional releases method).

.match() can also return a Promise of the result. It’s useful e.g. for filesystem checks.


Allows to implement simplified registry.

var locatedSource = resolver.locate( source )
  • source: String - source from bower.json, like jquery/jquery
  • locatedSource: String - Returns a resolved source string, like "git://github.com/jquery/jquery.git"

.locate() can also return a Promise of the result. It’s useful e.g. for remote registry calls.


Bower selects one matching version from the result and passes matching target field to fetch method.

var resolvedReleases = resolver.releases( source )
  • source: String - source from bower.json, like git://github.com/jquery/jquery.git
  • resolvedReleases: Array - Returns available releases for given source (like list of available tags on GitHub)
    • target: String - unique target id for release (usually tag name)
    • version: String - semantic version for the target above

.releases() can also return a Promise of the result.


Downloads given endpoint and returns path to temporary directory.

var fetched = resolver.fetch( endPoint, cached )
  • endpoint: Object - endpoint for the resource to download
    • name: String - name of resource (like jquery)
    • source: String - where to download resource from (like git://github.com/jquery/jquery.git)
    • target: String - the version or release of resource to download (like v1.0.0)
  • cached: Object - contains information about cached resource
    • endpoint: Object - endpoint of cached resource (the same format as above)
    • release: String - release of cached resource
    • releases: Array - the result of releases method
    • version: String - present cached resource has been resolved as version (like 1.0.0)
    • resolution: String - the “resolution” returned from previous fetch call for same resource
  • fetched: Object - Returned
    • tempPath: String - path to teporary directory with downloaded resource
    • removeIgnores: Boolean - tells whether bower should remove files ignores in bower.json.
    • resolution: Object - extra object that is saved in .bower.json and passed in cached field to the next fetch call. It can be used e.g. to download resources conditionally, for example by storing e-tag or last-modified time.

.fetch() can also return a Promise of the result.

If .fetch() returns undefined, then Bower re-uses cached package.