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@rollup/plugin-dynamic-import-vars

🍣 A rollup plugin to support variables in dynamic imports in Rollup.

function importLocale(locale) {
  return import(`./locales/${locale}.js`);
}

Requirements

This plugin requires an LTS Node version (v14.0.0+) and Rollup v1.20.0+.

Install

Using npm:

npm install @rollup/plugin-dynamic-import-vars --save-dev

Usage

Create a rollup.config.js configuration file and import the plugin:

import dynamicImportVars from '@rollup/plugin-dynamic-import-vars';

export default {
  plugins: [
    dynamicImportVars({
      // options
    })
  ]
};

Options

`include`

Type: String | Array[...String]
Default: []

Files to include in this plugin (default all).

`exclude`

Type: String | Array[...String]
Default: []

Files to exclude in this plugin (default none).

`errorWhenNoFilesFound`

Type: Boolean
Default: false

By default, the plugin will not throw errors when target files are not found. Setting this option to true will result in errors thrown when encountering files which don't exist.

⚠️ Important: Enabling this option when warnOnError is set to true will result in a warning and not an error

`warnOnError`

Type: Boolean
Default: false

By default, the plugin quits the build process when it encounters an error. If you set this option to true, it will throw a warning instead and leave the code untouched.

How it works

When a dynamic import contains a concatenated string, the variables of the string are replaced with a glob pattern. This glob pattern is evaluated during the build, and any files found are added to the rollup bundle. At runtime, the correct import is returned for the full concatenated string.

Some example patterns and the glob they produce:

`./locales/${locale}.js` -> './locales/*.js'

`./${folder}/${name}.js` -> './*/*.js'

`./module-${name}.js` -> './module-*.js'

`./modules-${name}/index.js` -> './modules-*/index.js'

'./locales/' + locale + '.js' -> './locales/*.js'

'./locales/' + locale + foo + bar + '.js' -> './locales/*.js'

'./locales/' + `${locale}.js` -> './locales/*.js'

'./locales/' + `${foo + bar}.js` -> './locales/*.js'

'./locales/'.concat(locale, '.js') -> './locales/*.js'

'./'.concat(folder, '/').concat(name, '.js') -> './*/*.js'

Code that looks like this:

function importLocale(locale) {
  return import(`./locales/${locale}.js`);
}

Is turned into:

function __variableDynamicImportRuntime__(path) {
  switch (path) {
    case './locales/en-GB.js':
      return import('./locales/en-GB.js');
    case './locales/en-US.js':
      return import('./locales/en-US.js');
    case './locales/nl-NL.js':
      return import('./locales/nl-NL.js');
    default:
      return new Promise(function (resolve, reject) {
        queueMicrotask(reject.bind(null, new Error('Unknown variable dynamic import: ' + path)));
      });
  }
}

function importLocale(locale) {
  return __variableDynamicImportRuntime__(`./locales/${locale}.js`);
}

Import Assertions

This plugin will keep your import assertions inside dynamic import statements intact.

// Refer to rollup-plugin-import-css https://github.com/jleeson/rollup-plugin-import-css
function importLocale(sheet) {
  return import(`./styles/${sheet}.css`, { assert: { type: 'css' } });
}

This is important e.g. in the context of rollup-plugin-import-css dealing with CSS imports, due to there still being an assertion, it will resolve the CSS import to a CSSStyleSheet, similar to native browser behavior.

Limitations

To know what to inject in the rollup bundle, we have to be able to do some static analysis on the code and make some assumptions about the possible imports. For example, if you use just a variable you could in theory import anything from your entire file system.

function importModule(path) {
  // who knows what will be imported here?
  return import(path);
}

To help static analysis, and to avoid possible foot guns, we are limited to a couple of rules:

Imports must start with `./` or `../`.

All imports must start relative to the importing file. The import should not start with a variable, an absolute path or a bare import:

// Not allowed
import(bar);
import(`${bar}.js`);
import(`/foo/${bar}.js`);
import(`some-library/${bar}.js`);

Imports must end with a file extension

A folder may contain files you don't intend to import. We, therefore, require imports to end with a file extension in the static parts of the import.

// Not allowed
import(`./foo/${bar}`);
// allowed
import(`./foo/${bar}.js`);

Imports to your own directory must specify a filename pattern

If you import your own directory you likely end up with files you did not intend to import, including your own module. It is therefore required to give a more specific filename pattern:

// not allowed
import(`./${foo}.js`);
// allowed
import(`./module-${foo}.js`);

Globs only go one level deep

When generating globs, each variable in the string is converted to a glob * with a maximum of one star per directory depth. This avoids unintentionally adding files from many directories to your import.

In the example below this generates ./foo/*/*.js and not ./foo/**/*.js.

import(`./foo/${x}${y}/${z}.js`);

Uh oh!