intl-messageformat throwing warnings

I’m currently building a small app using the basic rollup svelte template, and svelte-i18n, and rollup is currently throwing the following warning when I build with locale or dictionary imported.

rollup v1.23.1
bundles src/main.js → public\bundle.js...
LiveReload enabled
(!) `this` has been rewritten to `undefined`
https://rollupjs.org/guide/en/#error-this-is-undefined       
node_modules\intl-messageformat\lib\core.js
4: See the accompanying LICENSE file for terms.
5: */
6: var __extends = (this && this.__extends) || (function () {
                    ^
7:     var extendStatics = function (d, b) {
8:         extendStatics = Object.setPrototypeOf ||
...and 3 other occurrences
node_modules\intl-messageformat\lib\compiler.js
4: See the accompanying LICENSE file for terms.
5: */
6: var __extends = (this && this.__extends) || (function () {
                    ^
7:     var extendStatics = function (d, b) {
8:         extendStatics = Object.setPrototypeOf ||
...and 1 other occurrence

My package.json

{
  "name": "svelte-app",
  "version": "1.0.0",
  "devDependencies": {
    "npm-run-all": "^4.1.5",
    "rollup": "^1.23.0",
    "rollup-plugin-commonjs": "^10.0.0",
    "rollup-plugin-livereload": "^1.0.0",
    "rollup-plugin-node-resolve": "^5.2.0",
    "rollup-plugin-svelte": "^5.1.0",
    "rollup-plugin-terser": "^5.1.2",
    "svelte": "^3.12.0"
  },
  "dependencies": {
    "date-fns": "^2.4.1",
    "lodash": "^4.17.15",
    "sirv-cli": "^0.4.4",
    "svelte-i18n": "^1.1.2-beta",
    "svelte-routing": "^1.4.0"
  },
  "scripts": {
    "build": "rollup -c",
    "autobuild": "rollup -c -w",
    "dev": "run-p start:dev autobuild",
    "start": "sirv public --single",
    "start:dev": "sirv public --single --dev"
  }
}

Anyone have any insights here?

[ashour]

Oh, and there may be a related issue in the intl-messageformat repo. It seems that the var __extends = (this && this.__extends) || (function () { code is created by TypeScript, which is the language intl-messageformat is written in, I believe. Not 100% sure about this but wanted to share in case someone wants to do a bit more digging.

[ashour]

Hello @Sureiya. Yeah this seems to be a known issue with Rollup. To be consistent with native module implementations, Rollup sets this = undefined at the top level in modules. It also displays a warning if a module tries to use this at the top level. intl-messageformat has code that uses this at the top level, which upsets Rollup.

The intl-messageformat code seems to work just fine, however, and I’ve gotten rid of the warnings by using the suggestion in the Rollup documentation to provide the two culprit modules—node_modules\intl-messageformat\lib\core.js and node_modules\intl-messageformat\lib\compiler.js—with window instead of undefined as the this value. Since all our code runs in the browser, I’m assuming the two modules expect this = window. I’ve achieve the override in my rollup.config.js:

import svelte from 'rollup-plugin-svelte';
import resolve from 'rollup-plugin-node-resolve';
import commonjs from 'rollup-plugin-commonjs';
import livereload from 'rollup-plugin-livereload';
import { terser } from 'rollup-plugin-terser';

const production = !process.env.ROLLUP_WATCH;

export default {
    input: 'src/main.js',
    output: {
        sourcemap: true,
        format: 'iife',
        name: 'app',
        file: 'public/bundle.js',
    },

    // 👇🏽 The added `moduleContext` key is the relevant override here.

    moduleContext: (id) => {
        const thisAsWindowForModules = [
            'node_modules/intl-messageformat/lib/core.js',
            'node_modules/intl-messageformat/lib/compiler.js',
        ];

        if (thisAsWindowForModules.some(id_ => id.trimRight().endsWith(id_))) {
            return 'window';
        }
    },

   // 👆🏽

    plugins: [
        svelte({
            // enable run-time checks when not in production
            dev: !production,
            // we'll extract any component CSS out into
            // a separate file — better for performance
            css: css => {
                css.write('public/bundle.css');
            }
        }),

        // If you have external dependencies installed from
        // npm, you'll most likely need these plugins. In
        // some cases you'll need additional configuration —
        // consult the documentation for details:
        // https://github.com/rollup/rollup-plugin-commonjs
        resolve({
            browser: true,
            dedupe: importee => importee === 'svelte' || importee.startsWith('svelte/')
        }),
        commonjs(),

        // Watch the `public` directory and refresh the
        // browser on changes when not in production
        !production && livereload('public'),

        // If we're building for production (npm run build
        // instead of npm run dev), minify
        production && terser()
    ],
    watch: {
        clearScreen: false
    }
};

May be this should be addressed in the README doc? Happy to provide a PR if people agree.