解决
The resolving process is pretty simple. We distinguish three types of requests:
- absolute path:
require("/home/me/file")
require("C:\Home\me\file")
- relative path:
require("../src/file")
require("./file")
- module path:
require("module")
require("module/lib/file")
解决一个绝对路径
We first check if the path points to a directory. For a directory we need to find the main file in this directory. Therefore the main
field in the package.json
is joined to the path. If there is no package.json
or no main
field, index
is used as filename.
We have an absolute path to a file now. We try to append all extensions (configuration option resolve.extensions
). The first existing file is used as result.
解决一个相对路径
The context directory is the directory of the resource file that contains the require
statement. If there is no resource file the configuration option context
is used as context directory. (This can occur for entry points or with loader-generated files).
The relative path is joined to the context directory and the resulting absolute file is resolved according to "Resolving an absolute path".
解决一个模块路径
For resolving a module we first gather all search directories for modules from the context directory. This process is similar to the node.js resolving process, but the search directories are configurable with the configuration option resolve.modulesDirectories
. In addition to this the directories in the configuration option resolve.root
are prepended and directories in the configuration option resolve.fallback
are appended.
The module is looked up in each module directory and resolved according to "Resolving an absolute path". If the first match has no success, the second is tried and so on.
混叠
There is a configuration option resolve.alias
which renames modules.
When trying to "resolve a module path" the module name is matched to the resolve.alias
option and when there is a match it gets replaced with the alias.
缓存
Every filesystem access is cached so that multiple parallel or serial requests to the same thing are merged. In watching mode only changed files are removed from cache (the watcher knows which files got changed). In non-watching mode the cache is purged before every compilation.
不安全缓存
There is a configuration option resolve.unsafeCache
which boosts performance by aggressive caching.
Every resolve process is cached and isn't ever purged. This is correct in most cases, but incorrect in edge cases (what edge cases?).
上下文
When trying to resolve a [[context]] "Resolving an absolute path" ends when a directory is found.
加载器
For loaders the configuration options in resolveLoader
are used.
In addition to that when trying to "resolve a module path" all module name variations in the configuration option resolveLoader.moduleTemplates
are tried.
异步
The above description suggests a serial process, but in the implementation the process is completely asynchronous and parallel. This may causes more filesystem access than required.