A resolver is a library which helps in locating a module by its absolute path. A module can be required as a dependency from another module as:

import foo from 'path/to/module';
// or
require('path/to/module');

The dependency module can be from the application code or a third-party library. The resolver helps webpack find the module code that needs to be included in the bundle for every such require/import statement. webpack uses enhanced-resolve to resolve file paths while bundling modules.

Resolving rules in webpack

Using enhanced-resolve, webpack can resolve three kinds of file paths:

Absolute paths

import '/home/me/file';

import 'C:\\Users\\me\\file';

Since we already have the absolute path to the file, no further resolution is required.

Relative paths

import '../src/file1';
import './file2';

In this case, the directory of the resource file where the import or require occurs is taken to be the context directory. The relative path specified in the import/require is joined to this context path to produce the absolute path to the module.

Module paths

import 'module';
import 'module/lib/file';

Modules are searched for inside all directories specified in resolve.modules. You can replace the original module path by an alternate path by creating an alias for it using the resolve.alias configuration option.

Once the path is resolved based on the above rule, the resolver checks to see if the path points to a file or a directory. If the path points to a file:

• If the path has a file extension, then the file is bundled straightaway.
• Otherwise, the file extension is resolved using the resolve.extensions option, which tells the resolver which extensions are acceptable for resolution e.g. .js, .jsx.

If the path points to a folder, then the following steps are taken to find the right file with the right extension:

• If the folder contains a package.json file, then fields specified in resolve.mainFields configuration option are looked up in order, and the first such field in package.json determines the file path.
• If there is no package.json or if the resolve.mainFields do not return a valid path, file names specified in the resolve.mainFiles configuration option are looked for in order, to see if a matching filename exists in the imported/required directory .
• The file extension is then resolved in a similar way using the resolve.extensions option.

webpack provides reasonable defaults for these options depending on your build target.

Resolving Loaders

This follows the same rules as those specified for file resolution. But the resolveLoader configuration option can be used to have separate resolution rules for loaders.

Caching

Every filesystem access is cached, so that multiple parallel or serial requests to the same file occur faster. In watch mode, only modified files are evicted from the cache. If watch mode is off, then the cache gets purged before every compilation.

See Resolve API to learn more on the configuration options mentioned above.