Importing leaflet into module from cdn with typescript support-CodePudding

I'm trying to import leaflet into a javascript module with typescript support but can't get it to work.

I've installed @types/leaflet and have tried to following:
import 'https://cdnjs.cloudflare.com/ajax/libs/leaflet/1.8.0/leaflet.js'
This works fine in the browser but typescript gives the following error:
'L' refers to a UMD global, but the current file is a module. Consider adding an import instead.ts(2686)
(I call L.map() in my code)

I've also tried: import * as L from 'https://cdnjs.cloudflare.com/ajax/libs/leaflet/1.8.0/leaflet.js'
This works fine in typescript, but gives the following error in chrome:
Uncaught TypeError: L.map is not a function

Anyone know how to fix this?
I don't want to use a bundler.

Edit As jsejcksn suggested in the comments, importing a ES module version of leaflet and adding a path alias to typescript fixed this. I'm now using the following import:
import * as L from 'https://unpkg.com/[email protected]/dist/leaflet-src.esm.js'
The problem with this though, as stated on Leaflet's Download page, is that this imports Leaflet's source files including unit tests, files for debugging, build scripts, etc. This doesn't seem very efficient to me.
Any other ideas?

Edit 2 It seems like Leaflet 1.9 will solve my problem when it will be released. This pull request adds an ES module entrypoint to Leaflet meaning you can do this:
import L from 'leaflet'
or in our case:
import L from 'cdn.com/link/to/leaflet/1.9/esm'

In the mean time, jsejcksn's answer provides a good alternative so I will accept that answer.

CodePudding user response：

Preface

I've already written an answer about how to do this with ES modules at the question How to include modules in client side in typescript projects?. Be sure to read through that question and answer for context here.

You said:

I don't want to use a bundler.

and also:

The problem with this though, as stated on Leaflet's Download page, is that this imports Leaflet's source files including unit tests, files for debugging, build scripts, etc. This doesn't seem very efficient to me.

These ideas seem at odds to me. Without more information, I can only assume that by "not very efficient" you mean that the byte size of the presumably not-tree-shaken ES module is larger than the UMD module. If that's the case, why not just import Leaflet into your bundle and tree-shake away all of the parts that you don't use during build, resulting in even fewer bytes than the UMD?

Ignoring that for now, and responding purely to your stated criteria, below I will provide a self-contained, reproducible example demonstrating how you can use a CDN-hosted UMD module that has a corresponding @types package in a TypeScript ES module without augmenting the definition of globalThis.window in the compiler and without bundling.

Note that some of what I show will be purely ceremonial, but the technique will allow for you to later swap out the shim module URL in the import map with a CDN ES module URL if/when you find one that suits you.

Download the files in this example

I'm going to provide a complete reproducible example here, including all the content of each file in a code block. If you want to use this example to reproduce it and don't want to copy paste every individual code block into a new file with a matching name on your device, then you can copy paste the following script and run it in your browser console to download the repo as a zip archive file:

function download (data, fileName, mediaType) {
  const a = document.createElement('a');
  const blob = new Blob([data], mediaType ? {type: mediaType} : undefined);
  const url = URL.createObjectURL(blob);
  a.href = url;
  a.download = fileName;
  a.click();
  a.remove();
  // URL.revokeObjectURL(url);
}

download(
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  "so-73091042.zip",
  "application/zip",
);

Example

This example will be served using a static file server from the ./public directory, and the entrypoint html file is here:

./public/index.html:

<!doctype html>
<html lang="en">

<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />

  <title>Stack Overflow 73091042</title>

  <!-- First, define an import map for the "leaflet" module specifier
  which points to the UMD-to-ESM shim module: -->
  <script type="importmap">
    {
      "imports": {
        "leaflet": "/leaflet_umd_shim.js"
      }
    }    
  </script>

  <!-- Then, import the entrypoint module: -->
  <script type="module" src="/main.js"></script>
</head>

<body>
  <h1>See console output</h1>
</body>

</html>

Nothing special there except maybe the import map if you aren't familiar: WICG/import-maps: How to control the behavior of JavaScript imports

Next, there are two TypeScript source files in the ./src directory which are transpiled to JavaScript and copied into the ./public directory during compilation.

The first is a shim:

./src/leaflet_umd_shim.ts:

// Creates a proxy to the UMD Leaflet object `window.L` as a type-safe ESM export

/*

This loads (and executes) the UMD Leaflet script the first time that
this module is imported into the module graph, which assigns
the UMD Leaflet object to `window.L`. This is scope pollution, and is
one of the things that ES modules avoid, but is necessary here because
you expressed dissatisfaction at the available Leaflet ES module options
at existing CDN URLs.

This proxy technique "works" well here because Leaflet
is an object export/namspace and is documented to be used this way. However,
note that — for other modules which provide multiple exports that are
intended to be imported individually by name — this technique wouldn't be
a good fit because it requires the entire UMD module to be made available
on a single (named) export. (Note that the default export is just a named export
using the name "default").

*/

import 'https://cdnjs.cloudflare.com/ajax/libs/leaflet/1.8.0/leaflet.js';

import type * as Leaflet from 'leaflet';

const {L} = window as unknown as { L: typeof Leaflet };

// Same as `export {L as default}`:
export default L;

The "leaflet" import specifier in the import map is bound to the location of the JavaScript module produced by this file during compilation.

That specifier is used in the entrypoint module:

./src/main.ts:

/*

(Reference comments in "leaflet_umd_shim.ts" for context)
Example of non-working named import:

import {tileLayer} from 'leaflet';

It would throw the following runtime exception:
"Uncaught SyntaxError: The requested module 'leaflet' does not provide
an export named 'tileLayer'"

This technique requires importing the entire UMD module on a name (even if that
name is "default"):

*/

// Same as `import {default as L} from 'leaflet'`:
import L from 'leaflet';

const a = L.latLng(50.5, 30.5);
const b = L.latLng(51.5, 30.5);
const distance = a.distanceTo(b);

console.log({a, b, distance}); /* Prints the following:

{
  "a": {
    "lat": 50.5,
    "lng": 30.5
  },
  "b": {
    "lat": 51.5,
    "lng": 30.5
  },
  "distance": 111194.92664455874
}

*/

When you serve this site on a local http server and access the HTML file, the main module will execute and print that information to the console using those methods from Leaflet to create the data that is logged, proving that this technique works.

Here's my TSConfig:

./tsconfig.json:

{
  "compilerOptions": {
    // Strict options (This is TS: type safety is important)
    "exactOptionalPropertyTypes": true,
    "isolatedModules": true,
    "noUncheckedIndexedAccess": true,
    "strict": true,
    "useUnknownInCatchVariables": true,

    // ESM
    "esModuleInterop": true,
    "module": "esnext",
    "moduleResolution": "nodenext",

    "lib": [
      "esnext", // Latest ES syntax features
      "dom", // Browser runtime environmnt
      "dom.iterable" // Browser runtime environmnt
    ],

    "target": "esnext", // Adjust this to your browser target needs

    "outDir": "public", // Directory from which the site will be served

    // Path mapping
    // Ref: https://www.typescriptlang.org/docs/handbook/module-resolution.html#path-mapping
    "baseUrl": ".",
    "paths": {
      "leaflet": ["node_modules/@types/leaflet"]
    },

    // Optional settings
    "removeComments": true
  },
  "include": ["src/**/*"]
}

And, lastly, here's the npm package file:

./package.json:

{
  "name": "so-73091042",
  "version": "0.1.0",
  "type": "module",
  "scripts": {
    "compile": "tsc",
    "serve": "static --port=8000 --cache=false public"
  },
  "license": "MIT",
  "devDependencies": {
    "@types/leaflet": "^1.7.11",
    "node-static": "^0.7.11",
    "typescript": "^4.7.4"
  },
  "volta": {
    "node": "16.16.0"
  }
}

You can ignore the volta property: it just shows that I was using Volta and the version of Node that I had installed when creating this repository example.

The node-static dependency is the package recommended by the official Node.js website for serving static files in the article How to serve static files | Node.js.

First use npm install to install the dependencies:

% npm install

added 9 packages, and audited 10 packages in 4s

Then — to compile the TS files and emit them — use npm run compile:

% npm run compile

> [email protected] compile
> tsc

There should now be two new files in the ./public directory after compilation:

./public/leaflet_umd_shim.js:

import 'https://cdnjs.cloudflare.com/ajax/libs/leaflet/1.8.0/leaflet.js';
const { L } = window;
export default L;

./public/main.js:

import L from 'leaflet';
const a = L.latLng(50.5, 30.5);
const b = L.latLng(51.5, 30.5);
const distance = a.distanceTo(b);
console.log({ a, b, distance });

Now everything is ready to be served by the static file server, so use the command npm run serve:

% npm run serve 

> [email protected] serve
> static --port=8000 --cache=false public

serving "public" at http://127.0.0.1:8000

If you navigate to http://127.0.0.1:8000 in your browser (this is the same as http://localhost:8000), you'll see the index page with the text "See console output", and if you look at your browser's JS console, you'll see a representation of the object data that was logged to the console in the main.js module.

When you're ready to stop the server, just use ctrl c in your terminal

And that's it!