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105 lines
3.7 KiB
Markdown
105 lines
3.7 KiB
Markdown
# Linking to third party code
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In the [Getting Started](./getting_started.md) section, we saw Deno could
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execute scripts from URLs. Like browser JavaScript, Deno can import libraries
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directly from URLs. This example uses a URL to import an assertion library:
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```ts
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import { assertEquals } from "https://deno.land/std/testing/asserts.ts";
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assertEquals("hello", "hello");
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assertEquals("world", "world");
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console.log("Asserted! 🎉");
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```
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Try running this:
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```shell
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$ deno run test.ts
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Compile file:///mnt/f9/Projects/github.com/denoland/deno/docs/test.ts
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Download https://deno.land/std/testing/asserts.ts
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Download https://deno.land/std/fmt/colors.ts
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Download https://deno.land/std/testing/diff.ts
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Asserted! 🎉
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```
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Note that we did not have to provide the `--allow-net` flag for this program,
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and yet it accessed the network. The runtime has special access to download
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imports and cache them to disk.
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Deno caches remote imports in a special directory specified by the `$DENO_DIR`
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environment variable. It defaults to the system's cache directory if `$DENO_DIR`
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is not specified. The next time you run the program, no downloads will be made.
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If the program hasn't changed, it won't be recompiled either. The default
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directory is:
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- On Linux/Redox: `$XDG_CACHE_HOME/deno` or `$HOME/.cache/deno`
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- On Windows: `%LOCALAPPDATA%/deno` (`%LOCALAPPDATA%` = `FOLDERID_LocalAppData`)
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- On macOS: `$HOME/Library/Caches/deno`
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- If something fails, it falls back to `$HOME/.deno`
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## FAQ
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### How do I import a specific version of a module?
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Specify the version in the URL. For example, this URL fully specifies the code
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being run: `https://unpkg.com/liltest@0.0.5/dist/liltest.js`.
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### It seems unwieldy to import URLs everywhere.
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> What if one of the URLs links to a subtly different version of a library?
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> Isn't it error prone to maintain URLs everywhere in a large project?
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The solution is to import and re-export your external libraries in a central
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`deps.ts` file (which serves the same purpose as Node's `package.json` file).
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For example, let's say you were using the above assertion library across a large
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project. Rather than importing `"https://deno.land/std/testing/asserts.ts"`
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everywhere, you could create a `deps.ts` file that exports the third-party code:
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```ts
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export {
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assert,
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assertEquals,
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assertStrContains,
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} from "https://deno.land/std/testing/asserts.ts";
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```
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And throughout the same project, you can import from the `deps.ts` and avoid
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having many references to the same URL:
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```ts
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import { assertEquals, runTests, test } from "./deps.ts";
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```
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This design circumvents a plethora of complexity spawned by package management
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software, centralized code repositories, and superfluous file formats.
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### How can I trust a URL that may change?
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By using a lock file (with the `--lock` command line flag), you can ensure that
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the code pulled from a URL is the same as it was during initial development. You
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can learn more about this
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[here](./linking_to_external_code/integrity_checking.md).
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### But what if the host of the URL goes down? The source won't be available.
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This, like the above, is a problem faced by _any_ remote dependency system.
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Relying on external servers is convenient for development but brittle in
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production. Production software should always vendor its dependencies. In Node
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this is done by checking `node_modules` into source control. In Deno this is
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done by pointing `$DENO_DIR` to some project-local directory at runtime, and
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similarly checking that into source control:
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```shell
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# Download the dependencies.
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DENO_DIR=./deno_dir deno cache src/deps.ts
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# Make sure the variable is set for any command which invokes the cache.
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DENO_DIR=./deno_dir deno test src
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# Check the directory into source control.
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git add -u deno_dir
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git commit
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```
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