# Load a JavaScript module from a URL with IMPORT()

> Download a file with JavaScript from an address, keep it in the tmp folder, require it and use what it exports; the second call does not download it again.

Framework: total5
Product: Total.js 5 (total5@0.0.19)
Language: en
Content-SHA256: d9ee8c3000dc6dc3cbcd81c5a4523bfb6863dca95d50aafed96c8f1755a91c0a
Snapshot: c0700e321579725c5cf4f5ace1ad7d2866d1b4f93c24a63c0b41b6b7998e976c
Sources: total5-installed@total5@0.0.19
Review: not-recorded

`IMPORT(url, [callback])` downloads a JavaScript file, saves it in the `tmp` folder of the application and returns what the file exports (`module.exports`), as `require()` would. With a callback it is `callback(err, module, response)`; without it, `IMPORT()` returns a promise for the module. The address is remembered: calling `IMPORT()` again with the same address does **not** download the file again, it requires the one in `tmp` (and the file stays there between restarts, but the memory of "already downloaded" does not: after a restart it is downloaded again).

**This runs code that comes from the network with all the rights of your application.** Import only from an address that you control and that you reach with `https`, never one that a user sent you or that anyone else can change. The file is a CommonJS module. **The status of the answer is not checked:** an address that answers `404` (or any error page) is saved as if it were the module, and `require()` then throws a `SyntaxError` inside the download, so in `total5` 0.0.19 the callback is **never called** and the promise never settles. Always give `IMPORT()` a limit of time of your own, as the last route does, and check the address before you depend on it.

Here the file comes from a route of the same application, so that the example needs no network. The route counts the downloads.

```js controllers/import.js
exports.install = function() {
	ROUTE('GET /lib/tax/', lib);
	ROUTE('POST /use/', use);
	ROUTE('POST /use-twice/', use_twice);
	ROUTE('POST /missing/', missing);
	ROUTE('GET /downloads/', downloads);
};

let count = 0;

function lib($) {
	count++;
	$.plain('module.exports = { vat: function(price) { return Math.round(price * 121) / 100; } };');
}

function downloads($) {
	$.json({ downloads: count });
}

function base() {
	return 'http://127.0.0.1:' + CONF.$port;
}

async function use($) {
	const tax = await IMPORT(base() + '/lib/tax/');
	$.json({ price: tax.vat(100) });
}

function use_twice($) {
	IMPORT(base() + '/lib/tax/', function(err, tax) {
		$.json({ err: err ? err.toString() : null, price: tax ? tax.vat(200) : null });
	});
}

async function missing($) {
	// the address does not exist: the answer is the 404 page of the application
	const winner = await Promise.race([
		IMPORT(base() + '/lib/missing/').then(() => 'answered', () => 'answered'),
		new Promise(resolve => setTimeout(() => resolve('no answer'), 1500))
	]);
	$.json({ result: winner });
}
```