A webpack loader that executes a given module and returns the result of the execution at build-time, when the module is required in the bundle. In this way, the loader changes a module from code into a result.
Another way to view val-loader is that it allows users to implement custom loader logic without needing to write a full custom loader.
The target module is called with two arguments: (options, loaderContext)
options: The loader options (for instance provided in the webpack config. See the example below).loaderContext: The loader context.To begin, you'll need to install val-loader:
npm install val-loader --save-dev
Then, add the loader to your webpack configuration. For example:
target-file.js
module.exports = (options, loaderContext) => {
return { code: "module.exports = 42;" };
};
webpack.config.js
module.exports = {
module: {
rules: [
{
test: /target-file.js$/,
use: [
{
loader: `val-loader`,
},
],
},
],
},
};
src/entry.js
const answer = require("target-file");
Finally, run webpack using the method you normally use (e.g., via CLI or an npm script).
Type:
type executableFile = string;
Default: undefined
Allows to specify path to the executable file.
data.json
executable-file.js
module.exports = function yearsInMs(options, loaderContext, content) {
const { years } = JSON.parse(content);
const value = years * 365 * 24 * 60 * 60 * 1000;
return {
cacheable: true,
code: "module.exports = " + value,
};
};
webpack.config.js
module.exports = {
module: {
rules: [
{
test: /\.(json)$/i,
rules: [
{
loader: "val-loader",
options: {
executableFile: path.resolve(
__dirname,
"fixtures",
"executableFile.js",
),
},
},
],
},
{
test: /\.json$/i,
type: "asset/resource",
},
],
},
};
Targeted modules of this loader must export a function that returns an object, or a Promise resolving to an object (e.g. async function), containing a code property at a minimum, but can also include additional properties.
Type:
type code = string | Buffer;
Default: undefined Required
Code passed along to webpack or the next loader that will replace the original module.
Type:
Default: undefined
A source map passed along to webpack or the next loader.
Type:
type ast = Array<object>;
Default: undefined
An Abstract Syntax Tree (AST) that will be passed to the next loader. Useful to speed up the build time if the next loader uses the same AST.
Type:
type dependencies = Array<string>;
Default: []
An array of absolute, native paths to file dependencies that should be watched by webpack for changes.
Dependencies can also be added using loaderContext.addDependency(file: string).
Type:
type contextDependencies = Array<string>;
Default: []
An array of absolute, native paths to directory dependencies that should be watched by webpack for changes.
Context dependencies can also be added using loaderContext.addContextDependency(directory: string).
Type:
type buildDependencies = Array<string>;
Default: []
An array of absolute, native paths to directory dependencies that should be watched by webpack for changes.
Build dependencies can also be added using loaderContext.addBuildDependency(file: string).
Type:
type cacheable = boolean;
Default: false
If true, specifies that the code can be reused in watch mode if none of the dependencies have changed.
In this example the loader is configured to operate on a file name of years-in-ms.js, execute the code, and store the result in the bundle as the result of the execution. This example passes years as an option, which corresponds to the years parameter in the target module's exported function:
years-in-ms.js
module.exports = function yearsInMs({ years }) {
const value = years * 365 * 24 * 60 * 60 * 1000;
// NOTE: this return value will replace the module in the bundle
return {
cacheable: true,
code: "module.exports = " + value,
};
};
webpack.config.js
module.exports = {
module: {
rules: [
{
test: require.resolve("src/years-in-ms.js"),
use: [
{
loader: "val-loader",
options: {
years: 10,
},
},
],
},
],
},
};
In the bundle, requiring the module then returns:
import tenYearsMs from "years-in-ms"; console.log(tenYearsMs); // 315360000000
Example shows how to build modernizr.
entry.js
import modenizr from "./modernizr.js";
modernizr.js
const modernizr = require("modernizr");
module.exports = function (options) {
return new Promise(function (resolve) {
// It is impossible to throw an error because modernizr causes the process.exit(1)
modernizr.build(options, function (output) {
resolve({
cacheable: true,
code: `var modernizr; var hadGlobal = 'Modernizr' in window; var oldGlobal = window.Modernizr; ${output} modernizr = window.Modernizr; if (hadGlobal) { window.Modernizr = oldGlobal; } else { delete window.Modernizr; } export default modernizr;`,
});
});
});
};
webpack.config.js
const path = require("path");
module.exports = {
module: {
rules: [
{
test: path.resolve(__dirname, "src", "modernizr.js"),
use: [
{
loader: "val-loader",
options: {
minify: false,
options: ["setClasses"],
"feature-detects": [
"test/css/flexbox",
"test/es6/promises",
"test/serviceworker",
],
},
},
],
},
],
},
};
Example shows how to build figlet.
entry.js
import { default as figlet } from "./figlet.js";
console.log(figlet);
figlet.js
const figlet = require("figlet");
function wrapOutput(output, config) {
let figletOutput = "";
if (config.textBefore) {
figletOutput += encodeURI(`${config.textBefore}\n`);
}
output.split("\n").forEach((line) => {
figletOutput += encodeURI(`${line}\n`);
});
if (config.textAfter) {
figletOutput += encodeURI(`${config.textAfter}\n`);
}
return `module.exports = decodeURI("${figletOutput}");`;
}
module.exports = function (options) {
const defaultConfig = {
fontOptions: {
font: "ANSI Shadow",
horizontalLayout: "default",
kerning: "default",
verticalLayout: "default",
},
text: "FIGLET-LOADER",
textAfter: null,
textBefore: null,
};
const config = Object.assign({}, defaultConfig, options);
return new Promise(function (resolve, reject) {
figlet.text(config.text, config.fontOptions, (error, output) => {
if (error) {
return reject(error);
}
resolve({
cacheable: true,
code: "module.exports = " + wrapOutput(output, config),
});
});
});
};
webpack.config.js
const path = require("path");
module.exports = {
module: {
rules: [
{
test: path.resolve(__dirname, "src", "figlet.js"),
use: [
{
loader: "val-loader",
options: {
text: "FIGLET",
},
},
],
},
],
},
};
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