esm-dev/esm.sh - Githubissues

esm.sh

The no-build content delivery network(CDN) for modern web development.

How to Use

esm.sh allows you to import JavaScript modules from http URLs, no installation/build steps needed.

import * as mod from "https://esm.sh/PKG[@SEMVER][/PATH]";

With import maps, you can even use bare import specifiers intead of URLs:

<script type="importmap">
  {
    "imports": {
      "react": "https://esm.sh/react@18.2.0",
      "react-dom/": "https://esm.sh/react-dom@18.2.0/"
    }
  }
</script>
<script type="module">
  import React from "react"; // → https://esm.sh/react@18.2.0
  import { render } from "react-dom/client"; // → https://esm.sh/react-dom@18.2.0/client
</script>

More details about the import map usages can be found in the Using Import Maps section.

Supported Registries

NPM:

// Examples
import React from "https://esm.sh/react"; // latest
import React from "https://esm.sh/react@17"; // 17.0.2
import React from "https://esm.sh/react@beta"; // latest beta
import { renderToString } from "https://esm.sh/react-dom/server"; // sub-modules

JSR (starts with /jsr/):

// Examples
import { encodeBase64 } from "https://esm.sh/jsr/@std/encoding@1.0.0/base64";
import { Hono } from "https://esm.sh/jsr/@hono/hono@4";

GitHub (starts with /gh/):

// Examples
import tslib from "https://esm.sh/gh/microsoft/tslib"; // latest
import tslib from "https://esm.sh/gh/microsoft/tslib@2.6.0"; // the version '2.6.0' is a git tag

pkg.pr.new (starts with /pr/ or /pkg.pr.new/):

// Examples
import { Bench } from "https://esm.sh/pr/tinylibs/tinybench/tinybench@a832a55";
import { Bench } from "https://esm.sh/pr/tinybench@a832a55"; // --compact

Specifying Dependencies

By default, esm.sh rewrites import specifiers based on the package dependencies. To specify the version of these dependencies, you can add ?deps=PACKAGE@VERSION to the import URL. To specify multiple dependencies, separate them with commas, like this: ?deps=react@17.0.2,react-dom@17.0.2.

import React from "https://esm.sh/react@17.0.2";
import useSWR from "https://esm.sh/swr?deps=react@17.0.2";

Aliasing Dependencies

You can also alias dependencies by adding ?alias=PACKAGE:ALIAS to the import URL. This is useful when you want to use a different package for a dependency.

import useSWR from "https://esm.sh/swr?alias=react:preact/compat";

in combination with ?deps:

import useSWR from "https://esm.sh/swr?alias=react:preact/compat&deps=preact@10.5.14";

Tree Shaking

By default, esm.sh exports a module with all its exported members. However, if you want to import only a specific set of members, you can specify them by adding a ?exports=foo,bar query to the import statement.

import { __await, __rest } from "https://esm.sh/tslib"; // 7.3KB
import { __await, __rest } from "https://esm.sh/tslib?exports=__await,__rest"; // 489B

By using this feature, you can take advantage of tree shaking with esbuild and achieve a smaller bundle size. Note, this feature doesn't work with CommonJS modules.

Bundling Strategy

By default, esm.sh bundles sub-modules that ain't declared in the exports field.

Bundling sub-modules can reduce the number of network requests for performance. However, it may bundle shared modules repeatedly. In extreme case, it may break the side effects of the package, or change the import.meta.url semantics. To avoid this, you can add ?bundle=false to disable the default bundling behavior:

import "https://esm.sh/@pyscript/core?bundle=false";

For package authors, you can override the bundling strategy by adding the esm.sh field to package.json:

{
  "name": "foo",
  "esm.sh": {
    "bundle": false // disables the default bundling behavior
  }
}

esm.sh also supports ?bundle=all query to bundle the module with all external dependencies(except in peerDependencies) into a single JS file.

import { Button } from "https://esm.sh/antd?bundle=all";

Development Mode

import React from "https://esm.sh/react?dev";

With the ?dev option, esm.sh builds a module with process.env.NODE_ENV set to "development" or based on the condition development in the exports field. This is useful for libraries that have different behavior in development and production. For example, React uses a different warning message in development mode.

ESBuild Options

By default, esm.sh checks the User-Agent header to determine the build target. You can also specify the target by adding ?target, available targets are: es2015 - es2022, esnext, deno, denonext, and node.

import React from "https://esm.sh/react?target=esnext";

Other supported options of esbuild:

Conditions

import foo from "https://esm.sh/foo?conditions=custom1,custom2";

Keep names

import foo from "https://esm.sh/foo?keep-names";

Ignore annotations

import foo from "https://esm.sh/foo?ignore-annotations";

Web Worker

esm.sh supports ?worker query to load the module as a web worker:

import createWorker from "https://esm.sh/monaco-editor/esm/vs/editor/editor.worker?worker";

// create a worker
const worker = createWorker();
// rename the worker by adding the `name` option for debugging
const worker = createWorker({ name: "editor.worker" });
// inject code into the worker
const worker = createWorker({ inject: "self.onmessage = (e) => self.postMessage(e.data)" });

You can import any module as a worker from esm.sh with the ?worker query. Plus, you can access the module's exports in the inject code. For example, uing the xxhash-wasm to hash strings in a worker:

import createWorker from "https://esm.sh/xxhash-wasm@1.0.2?worker";

// variable '$module' is the imported 'xxhash-wasm' module
const inject = `
const { default: xxhash } = $module
self.onmessage = async (e) => {
  const hasher = await xxhash()
  self.postMessage(hasher.h64ToString(e.data))
}
`;
const worker = createWorker({ inject });
worker.onmessage = (e) => console.log("hash is", e.data);
worker.postMessage("The string that is being hashed");

Note: The inject parameter must be a valid JavaScript code, and it will be executed in the worker context.

Package CSS

<link rel="stylesheet" href="https://esm.sh/monaco-editor?css">

This only works when the package imports CSS files in JS directly.

Importing WASM as Module

esm.sh supports importing wasm modules in JS directly, to do that, you need to add ?module query to the import URL:

import wasm from "https://esm.sh/@dqbd/tiktoken@1.0.3/tiktoken_bg.wasm?module";

const { exports } = new WebAssembly.Instance(wasm, imports);

Note: The *.wasm?module pattern requires the top-level-await feature to be supported by the browser.

Using Import Maps

Import Maps has been supported by most modern browsers and Deno natively. This allows bare import specifiers, such as import React from "react", to work.

esm.sh introduces the ?external for specifying external dependencies. By employing this query, esm.sh maintains the import specifier intact, leaving it to the browser/Deno to resolve based on the import map. For example:

<script type="importmap">
{
  "imports": {
    "preact": "https://esm.sh/preact@10.7.2",
    "preact/": "https://esm.sh/preact@10.7.2/",
    "preact-render-to-string": "https://esm.sh/preact-render-to-string@5.2.0?external=preact"
  }
}
</script>
<script type="module">
  import { h } from "preact";
  import { useState } from "preact/hooks";
  import { render } from "preact-render-to-string";
</script>

Alternatively, you can mark all dependencies as external by adding a * prefix before the package name:

{
  "imports": {
    "preact": "https://esm.sh/preact@10.7.2",
    "preact-render-to-string": "https://esm.sh/*preact-render-to-string@5.2.0",
    "swr": "https://esm.sh/*swr@1.3.0",
    "react": "https://esm.sh/preact@10.7.2/compat"
  }
}

Import maps supports trailing slash that can not work with URL search params friendly. To fix this issue, esm.sh provides a special format for import URL that allows you to use query params with trailing slash: change the query prefix ? to & and put it after the package version.

{
  "imports": {
    "react-dom": "https://esm.sh/react-dom@18.2.0?dev",
    "react-dom/": "https://esm.sh/react-dom@18.2.0&dev/"
  }
}

Escape Hatch: Raw Source Files

In rare cases, you may want to request JS source files from packages, as-is, without transformation into ES modules. To do so, you need to add a ?raw query to the request URL.

<script src="https://esm.sh/p5/lib/p5.min.js?raw"></script>

You may alternatively use raw.esm.sh/<PATH> as the origin, which is equivalent to esm.sh/<PATH>?raw, that transitive references in the raw assets will also be raw requests.

Deno Compatibility

esm.sh is a Deno-friendly CDN that resolves Node's built-in modules (such as fs, os, net, etc.), making it compatible with Deno.

import express from "https://esm.sh/express";

const app = express();
app.get("/", (req, res) => {
  res.send("Hello World");
});
app.listen(3000);

Deno supports type definitions for modules with a types field in their package.json file through the X-TypeScript-Types header. This makes it possible to have type checking and auto-completion when using those modules in Deno. (link).

Figure #1

In case the type definitions provided by the X-TypeScript-Types header is incorrect, you can disable it by adding the ?no-dts query to the module import URL:

import unescape from "https://esm.sh/lodash/unescape?no-dts";

This will prevent the X-TypeScript-Types header from being included in the network request, and you can manually specify the types for the imported module.

Supporting Node.js/Bun

Node.js and Bun don't currently support http modules.

We highly recommend Reejs as the runtime with esm.sh that works both in Node.js and Bun.

Global CDN

The Global CDN of esm.sh is provided by Cloudflare, one of the world's largest and fastest cloud network platforms.

Self-Hosting

To host esm.sh by yourself, check the hosting documentation.

esm-dev / esm.sh

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