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Explain how to test Deno KV code locally #313

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Explain how to test Deno KV code locally #313

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Explain how to test Deno KV code locally
NoraCodes Feb 4, 2024
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Update deploy/kv/manual/index.mdx
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Explain how to test Deno KV code locally 
This is easily apparent to users who are already familiar with SQLite,
but people who either don't know about ":memory:" or who aren't aware
that Deno KV is based on SQLite when run locally may not be able to
figure this out for themselves.
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commit ddb6f1925c3170f4fd1af686362b170c26c2d73a
39 changes: 39 additions & 0 deletions deploy/kv/manual/index.mdx
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KV on Deploy - a new Deploy database will be provisioned for you when required
by your code. Learn more about Deno KV on Deno Deploy [here](./on_deploy.mdx).

## Testing

By default, [`Deno.openKv()`](https://www.sqlite.org/inmemorydb.html) creates or
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opens a persistent store based on the path from which the script that invoked it
was run. This isn't usually desireable for tests, which need to produce the same
behavior when run many times in a row.

To test code that uses Deno KV, you can use the special argument `":memory:"` to
create an ephemeral Deno KV datastore.

```ts
async function setDisplayName(kv: Deno.Kv, username: string, displayname: string) {
await kv.set(["preferences", username, "displayname"], displayname);
}

async function getDisplayName(kv: Deno.Kv, username: string): Promise<string | null> {
return (await kv.get(["preferences", username, "displayname"])).value as string;
}

Deno.test("Preferences", async (t) => {
const kv = await Deno.openKv(":memory:");

await t.step("can set displayname", async () => {
const displayName = await getDisplayName(kv, "example");
assertEquals(displayName, null);

await setDisplayName(kv, "example", "Exemplary User");

const displayName = await getDisplayName(kv, "example");
assertEquals(displayName, "Exemplary User");
});
});
```

This works becaus Deno KV is backed by SQLite when run for local development. Just like
in-memory SQLite databases, multiple ephemeral Deno KV stores can exist at once without
interfering with one another. For more information about special database addressing modes,
see [the SQLite docs on the topic](https://www.sqlite.org/inmemorydb.html).

## Next steps

At this point, you're just beginning to scratch the surface with Deno KV. Be
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