1.2.8 • Public • Published


Convert NPM packages (ex: react, vue, d3, ...) into fresh ESM, locally!

Import them in any ESM environment (browser, Node.js v14+, Deno, etc).

Consuming react as a local package.

How it works

npm install -g uimport
  • UImport uses esbuild to generate the pre-packaged version of the bundle, and to obtain the metafile information of the package/subpath required to analyze the dependencies, esbuild is written in Go and it performs 10-100x faster than other bundlers written in javascript.

  • UImport does not install the packages, but instead requires the packages to be installed in the working directory before calling it.


We define as bundles to the ES modules generated from the packages or the subpaths of the packages.

By instance

react is a package that does not define subpaths, so react is a bundle.

svelte is a package that does define subpaths, so svelte/internal or svelte/store are bundles of the same svelte package.


UImport generates the packages considering the subpaths specified in the .exports property of the package.json. Specifying the .exports property of the package.json is a best practice for separating the package into independently units of code.

// The following two EM modules belong to the same package (svelte)
import 'svelte/internal';
import 'svelte/store';

When uimport generates a bundle, and finds a dependency on a subpath of a package that does not specify the .exports property, then uimport will not split the code, but rather include the code in the requested bundle.

Unfortunately, there is a wide variety of packages that do not follow such specific criteria. Many of them were created before the .exports property of package.json existed, and were generally intended to work with bundlers like webpack, and not to be consumed directly from a browser or deno.

Local server

Your application can consume the bundles through a local server provided by uimport, by running:

uimport server --port=8080 --cwd=working_directory

Resources URLs

Load the package at the version specified in the package.json.

import 'http://localhost:port/package_name.js';
import 'http://localhost:port/package_name/subpath.js';

Load the exact version of the package

We recommend importing packages by specifying their version.

import 'http://localhost:port/package_name@version.js';
import 'http://localhost:port/package_name@version/subpath.js';

Although an application uses a specific version of a package (according to what is specified in the package.json of your app), the packages have in turn its own dependencies and there could be version conflict among packages.

UImport generates by default the dependencies of the packages according to the version that each of them requires. This way, if your app specified a dependency (ex: dep_a@2.0.0) in the package.json, but then another dependency requires dep_a@1.0.0, uimport will be able to generate both.


UImport will look into the node_modules folder to find the package to be bundled. You can optionally specify the current working directory where node_modules resides.

const uimport = require('uimport');
const bundle = 'react';
const {code, errors, warnings, dependencies} = await uimport(bundle);
// mode can be 'esm', 'sjs', or 'amd'
const {code, errors, warnings, dependencies} = await uimport(bundle, mode, specs);
  • specs:
interface specs {
    cwd?: string,   // The working directory where the local NPM modules are installed
    temp?: string,  // A required folder to save temporary files. Default: join(cwd, '.uimport/temp')
    cache?: string, // Bundles are saved in cache. Default: join(cwd, '.uimport/cache');
    dependencies?: boolean // Build the dependencies of the bundle or not
interface output {
    code: string,
    errors?: string[],
    warnings?: string[],
    dependencies?: string[]

Pre-generated bundles and static resources

NPM packages were born essentially in order to host packages to be consumed by nodejs applications. Its use has spread and currently the packages hosted on NPM can be consumed by environments such as the Browser or Deno thanks to CDNs such as skypack, jsdelivr, jspm, unpkg.

However, package compatibility is dependent on a process of packaging and converting to ES modules. In the future this could change, simply if the authors specify in the package.json the input to the pre-built ES modules.

UImport currently skips the bundle generation process if it finds the uimport property in the package.json.

UImport property

Package.json example:

  "name": "your-package",
  "uimport": {
    "bundles": {
      "./store": "./store/index.js"
interface uimport {
    bundles: Record<string, string>,
    assets: Record<string, string>


  • .bundles Specifies the ES module to be returned for each entry point.
  • .assets Similar to the .bundles property, but defining general purpose files such as images.

About uimport

UImport was created and it is being maintained by the BeyondJS team. BeyondJS is an open source fullstack universal typescript framework, based on a modular project creation design where each project can be published (or not) in NPM packages that can in turn be universally consumed by any ESM environment.


We have observed that CDNs like skypack, jsdelivr, jspm or unpkg, as well as frameworks like vite are implementing their own NPM package packaging strategy. We believe that there should be an open source solution that should be easy to use and embraced by the developer community.

We have seen that the results of packages generated by CDNs in multiple cases vary. In some cases, some packages directly return an error on one CDN, and the processed code on another CDN.

CDNs are very important in our opinion. They can for example speed up the web thanks to the cross site cache. More and more client libraries emerge (such as vue, svelte, react, etc), they are widely used, but generally hosted individually in each project.

Feedback and collaboration

  • If you know of an alternative solution to uimport, please let us know.
  • If uimport is useful to you, maybe you can collaborate with us to join efforts.



npm i uimport

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