How to import javascript modules and bundle into a single js file with rollup?

I have javascript functions I want to run both server-side and client side. The server side uses node.js and can handle imports just fine.

However, the client side i want a single script tag with inlined javascript. To achieve that goal I understood that I needed a bundler to pull the modules into the main script file. I chose rollup to do that job.

Here's an example of what I've tried. I've made a module test.js. it's a simple function that returns a string:

// test.js

const test = () => {
    return 'testing'
}

module.exports = test

and here's the main javascript file, main.js. The file I want to pull the test.js function into:

// main.js

import test from "/test.js"

console.log(test())

Here's what i'd like main.js to look like after bundling:

// main.js

const test = () => {
    return 'testing'
}

console.log(test())

I'd like to simply pull the function into the file.

Instead rollup produces this:

(function (global, factory) {
    typeof exports === 'object' && typeof module !== 'undefined' ? factory(require('../../../../../../../js/test.js')) :
    typeof define === 'function' && define.amd ? define(['../../../../../../../js/test'], factory) :
    (global = typeof globalThis !== 'undefined' ? globalThis : global || self, factory(global.test));
}(this, (function (test) { 'use strict';

    function _interopDefaultLegacy (e) { return e && typeof e === 'object' && 'default' in e ? e : { 'default': e }; }

    var test__default = /*#__PURE__*/_interopDefaultLegacy(test);

    console.log(test__default['default']());

})));

The code is much larger than before, and crucially the module isn't incorporated into the main file. It's still a module, but with different syntax. Rollup has a number of output options, all produce similar results.

Here's my rollup.config.js file:

// rollup.config.js

export default [
    {
        input: 'src/scripts/main.js',
        output: {
            name: 'main',
            file: "public/scripts/main.js",
            format: 'umd'
        }
    }
]

Of course, this is the expected output for somebody who understands what's going on. But I don't understand.

How do I merge a javascript into one file at build time?

is there a simpler solution I'm overlooking?

Edit: Using the commonjs module and setting the output format to iife results in the following:

// main.js

(function (test) {
    'use strict';

    function _interopDefaultLegacy (e) { return e && typeof e === 'object' && 'default' in e ? e : { 'default': e }; }

    var test__default = /*#__PURE__*/_interopDefaultLegacy(test);

    console.log(test__default['default']());

}(test));

It's different, but the result still doesn't contain the test function.

You'll want to change the format to iife (Immediately Invoked Function Expression).

Your new rollup.config.js should look like this:

// rollup.config.js

export default [
    {
        input: 'src/scripts/main.js',
        output: {
            name: 'main',
            file: "public/scripts/main.js",
            format: 'iife'
        }
    }
]

If you also need to use CommonJS exports (module.exports = ___ instead of ES6 export default ___) you'll need to install @rollup/plugin-commonjs:

$ npm install --save-dev @rollup/plugin-commonjs

And use it like so:

import commonjs from '@rollup/plugin-commonjs'

export default [
    {
        input: 'src/scripts/main.js',
        plugins: [commonjs()],
        output: {
            name: 'main',
            file: "public/scripts/main.js",
            format: 'iife'
        }
    }
]

On a side note, make sure your import paths are correct.

This:

import test from "/test.js"

Should be:

import test from "./test.js"

You can find more info in the official docs.

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