A loader is a node module exporting a function
.
This function is called when a resource should be transformed by this loader.
In the simple case, when only a single loader is applied to the resource, the loader is called with one parameter: the content of the resource file as string.
The loader can access the loader API on the this
context in the function.
A sync loader that only wants to give a one value can simply return
it. In every other case the loader can give back any number of values with the this.callback(err, values...)
function. Errors are passed to the this.callback
function or thrown in a sync loader.
The loader is expected to give back one or two values. The first value is a resulting JavaScript code as string or buffer. The second optional value is a SourceMap as JavaScript object.
In the complex case, when multiple loaders are chained, only the last loader gets the resource file and only the first loader is expected to give back one or two values (JavaScript and SourceMap). Values that any other loader give back are passed to the previous loader.
Examples
// Identity loader module.exports = function(source) { return source; };
// Identity loader with SourceMap support module.exports = function(source, map) { this.callback(null, source, map); };
Guidelines
(Ordered by priority, first one should get the highest priority)
Loaders should
do only a single task
Loaders can be chained. Create loaders for every step, instead of a loader that does everything at once.
This also means they should not convert to JavaScript if not necessary.
Example: Render HTML from a template file by applying the query parameters
I could write a loader that compiles the template from source, execute it and return a module that exports a string containing the HTML code. This is bad.
Instead I should write loaders for every task in this use case and apply them all (pipeline):
- jade-loader: Convert template to a module that exports a function.
- apply-loader: Takes a function exporting module and returns raw result by applying query parameters.
- html-loader: Takes HTML and exports a string exporting module.
generate modules that are modular
Loader generated modules should respect the same design principles like normal modules.
Example: That?s a bad design: (not modular, global state, ?)
require("any-template-language-loader!./xyz.atl"); var html = anyTemplateLanguage.render("xyz");
flag itself cacheable if possible
Most loaders are cacheable, so they should flag itself as cacheable.
Just call cacheable
in the loader.
// Cacheable identity loader module.exports = function(source) { this.cacheable(); return source; };
not keep state between runs and modules
A loader should be independent of other modules compiled (expect of these issued by the loader).
A loader should be independent of previous compilations of the same module.
mark dependencies
If a loader uses external resources (i. e. by reading from filesystem), they must tell about that. This information is used to invalidate cacheable loaders and recompile in watch mode.
// Loader adding a header var path = require("path"); module.exports = function(source) { this.cacheable(); var callback = this.async(); var headerPath = path.resolve("header.js"); this.addDependency(headerPath); fs.readFile(headerPath, "utf-8", function(err, header) { if(err) return callback(err); callback(null, header + "\n" + source); }); };
resolve dependencies
In many languages there is some schema to specify dependencies. i. e. in css there is @import
and url(...)
. These dependencies should be resolved by the module system.
There are two options to do this:
- Transform them to
require
s. - Use the
this.resolve
function to resolve the path
Example 1 css-loader: The css-loader transform dependencies to require
s, by replacing @import
s with a require to the other stylesheet (processed with the css-loader too) and url(...)
with a require
to the referenced file.
Example 2 less-loader: The less-loader cannot transform @import
s to require
s, because all less files need to be compiled in one pass to track variables and mixins. Therefore the less-loader extends the less compiler with a custom path resolving logic. This custom logic uses this.resolve
to resolve the file with the configuration of the module system (aliasing, custom module directories, etc.).
If the language only accept relative urls (like css: url(file)
always means ./file
), there is the ~
-convection to specify references to modules:
url(file) -> require("./file") url(~module) -> require("module")
extract common code
don?t generate much code that is common in every module processed by that loader. Create a (runtime) file in the loader and generate a require
to that common code.
should not embed absolute paths
don?t put absolute paths in to the module code. They break hashing when the root for the project is moved. There is a method stringifyRequest
in loader-utils which converts an absolute path to an relative one.
Example:
var loaderUtils = require("loader-utils"); return "var runtime = require(" + loaderUtils.stringifyRequest(this, "!" + require.resolve("module/runtime")) + ");";
use a library as peerDependencies
when they wrap it
using a peerDependency allows the application developer to specify the exact version in package.json
if desired. The dependency should be relatively open to allow updating the library without needing to publish a new loader version.
"peerDependencies": { "library": "^1.3.5" }
programmable objects as query
-option
there are situations where your loader requires programmable objects with functions which cannot stringified as query
-string. The less-loader, for example, provides the possibility to specify LESS-plugins. In these cases, a loader is allowed to extend webpack?s options
-object to retrieve that specific option. In order to avoid name collisions, however, it is important that the option is namespaced under the loader?s camelCased npm-name.
Example:
// webpack.config.js module.exports = { ... lessLoader: { lessPlugins: [ new LessPluginCleanCSS({advanced: true}) ] } };
The loader should also allow to specify the config-key (e.g. lessLoader
) via query
. See discussion and example implementation.
be added to the list of loaders
Read more
Read more about loaders.
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