This example demonstrates how to build a library with webpack that has dependencies on other libraries which should not be included in the compiled version.

We use the `libraryTarget: "umd"` option to build a UMD module that is consumable in CommonJS, AMD and with script tags. We don't specify the `library` option so the library is exported to the root namespace.

We use the `externals` option to define dependencies that should be resolved in the target environment.

In the simple case we just need to specify a string (`"add"`). Then it's resolved as `"add"` module in CommonJS and AMD, and as global `add` when used with the script tag.

In the complex case we specify different values for each environment:

| environment        | config value             | resolved as                  |
| ------------------ | ------------------------ | ---------------------------- |
| CommonJS (strict)  | `["./math", "subtract"]` | `require("./math").subtract` |
| CommonJS (node.js) | `"./subtract"`           | `require("./subtract")`      |
| AMD                | `"subtract"`             | `define(["subtract"], ...)`  |
| script tag         | `"subtract"`             | `this.subtract`              |

# example.js

```javascript
var add = require("add");
var subtract = require("subtract");

exports.exampleValue = subtract(add(42, 2), 2);
```

# webpack.config.js

```javascript
"use strict";

/** @type {import("webpack").Configuration} */
const config = {
	// mode: "development" || "production",
	output: {
		libraryTarget: "umd"
	},
	externals: [
		"add",
		{
			subtract: {
				root: "subtract",
				commonjs2: "./subtract",
				commonjs: ["./math", "subtract"],
				amd: "subtract"
			}
		}
	]
};

module.exports = config;
```

# dist/output.js

```javascript
(function webpackUniversalModuleDefinition(root, factory) {
	if(typeof exports === 'object' && typeof module === 'object')
		module.exports = factory(require("add"), require("./subtract"));
	else if(typeof define === 'function' && define.amd)
		define(["add", "subtract"], factory);
	else {
		var a = typeof exports === 'object' ? factory(require("add"), require("./math")["subtract"]) : factory(root["add"], root["subtract"]);
		for(var i in a) (typeof exports === 'object' ? exports : root)[i] = a[i];
	}
})(self, (__WEBPACK_EXTERNAL_MODULE__1__, __WEBPACK_EXTERNAL_MODULE__2__) => {
return /******/ (() => { // webpackBootstrap
/******/ 	var __webpack_modules__ = ([
/* 0 */,
/* 1 */
/*!**********************!*\
  !*** external "add" ***!
  \**********************/
/*! dynamic exports */
/*! exports [maybe provided (runtime-defined)] [no usage info] */
/*! runtime requirements: module */
/***/ ((module) => {

"use strict";
module.exports = __WEBPACK_EXTERNAL_MODULE__1__;

/***/ }),
/* 2 */
/*!***************************************************************************************************************!*\
  !*** external {"root":"subtract","commonjs2":"./subtract","commonjs":["./math","subtract"],"amd":"subtract"} ***!
  \***************************************************************************************************************/
/*! dynamic exports */
/*! exports [maybe provided (runtime-defined)] [no usage info] */
/*! runtime requirements: module */
/***/ ((module) => {

"use strict";
module.exports = __WEBPACK_EXTERNAL_MODULE__2__;

/***/ })
/******/ 	]);
```

<details><summary><code>/* webpack runtime code */</code></summary>

``` js
/************************************************************************/
/******/ 	// The module cache
/******/ 	const __webpack_module_cache__ = {};
/******/ 	
/******/ 	// The require function
/******/ 	function __webpack_require__(moduleId) {
/******/ 		// Check if module is in cache
/******/ 		const cachedModule = __webpack_module_cache__[moduleId];
/******/ 		if (cachedModule !== undefined) {
/******/ 			return cachedModule.exports;
/******/ 		}
/******/ 		// Create a new module (and put it into the cache)
/******/ 		const module = __webpack_module_cache__[moduleId] = {
/******/ 			// no module.id needed
/******/ 			// no module.loaded needed
/******/ 			exports: {}
/******/ 		};
/******/ 	
/******/ 		// Execute the module function
/******/ 		__webpack_modules__[moduleId](module, module.exports, __webpack_require__);
/******/ 	
/******/ 		// Return the exports of the module
/******/ 		return module.exports;
/******/ 	}
/******/ 	
/************************************************************************/
```

</details>

``` js
let __webpack_exports__ = {};
// This entry needs to be wrapped in an IIFE because it needs to be isolated against other modules in the chunk.
(() => {
let exports = __webpack_exports__;
/*!********************!*\
  !*** ./example.js ***!
  \********************/
/*! default exports */
/*! export exampleValue [provided] [maybe used in main (runtime-defined)] [usage prevents renaming] */
/*! other exports [not provided] [maybe used in main (runtime-defined)] */
/*! runtime requirements: __webpack_exports__, __webpack_require__ */
var add = __webpack_require__(/*! add */ 1);
var subtract = __webpack_require__(/*! subtract */ 2);

exports.exampleValue = subtract(add(42, 2), 2);
})();

/******/ 	return __webpack_exports__;
/******/ })()
;
});
```

# Info

## Unoptimized

```
asset output.js 3.28 KiB [emitted] (name: main)
chunk (runtime: main) output.js (main) 194 bytes [entry] [rendered]
  > ./example.js main
  dependent modules 84 bytes [dependent] 2 modules
  ./example.js 110 bytes [built] [code generated]
    [exports: exampleValue]
    [used exports unknown]
    entry ./example.js main
    used as library export
webpack X.X.X compiled successfully
```

## Production mode

```
asset output.js 675 bytes [emitted] [minimized] (name: main)
chunk (runtime: main) output.js (main) 194 bytes [entry] [rendered]
  > ./example.js main
  dependent modules 84 bytes [dependent] 2 modules
  ./example.js 110 bytes [built] [code generated]
    [exports: exampleValue]
    entry ./example.js main
    used as library export
webpack X.X.X compiled successfully
```
