Deno KV (built-in key-value store)

Introduction

Deno KV lets you save and get data quickly using simple keys. It works like a small notebook where you write and read notes easily.

You want to save user settings in a small app.

You need to store simple data between program runs.

You want to keep track of simple counters or flags.

You want a quick way to save and load data without a big database.

Syntax

Deno

const kv = await Deno.openKv();

// To set a value
await kv.set(["key"], value);

// To get a value
const result = await kv.get(["key"]);
console.log(result.value);

Keys are arrays to allow grouping, like folders and files.

Values can be any JSON-serializable data.

Examples

Save and get a simple string value.

Deno

await kv.set(["username"], "alice");
const user = await kv.get(["username"]);
console.log(user.value);

Use nested keys to organize data like folders.

Deno

await kv.set(["settings", "theme"], "dark");
const theme = await kv.get(["settings", "theme"]);
console.log(theme.value);

Store and retrieve a number.

Deno

await kv.set(["counter"], 5);
const count = await kv.get(["counter"]);
console.log(count.value);

Sample Program

This program saves a user's name and age in Deno KV, then reads and prints them.

Deno

async function demoKv() {
  const kv = await Deno.openKv();

  // Save a user's name
  await kv.set(["user", "name"], "Bob");

  // Save user's age
  await kv.set(["user", "age"], 30);

  // Get and print the name
  const nameResult = await kv.get(["user", "name"]);
  console.log(`Name: ${nameResult.value}`);

  // Get and print the age
  const ageResult = await kv.get(["user", "age"]);
  console.log(`Age: ${ageResult.value}`);
}

demoKv();

Important Notes

Always await the set and get calls because they work with promises.

If a key does not exist, get() returns an object with value undefined.

Keys are arrays to help organize data in groups.

Summary

Deno KV stores data using keys that are arrays.

You use set to save and get to read data.

It is simple and good for small, quick data storage needs.

Practice

1. What is the correct way to store a value in Deno KV?

easy

A. Use kv.set(["key"], value) to save data.

B. Use kv.save("key", value) to save data.

C. Use kv.put("key", value) to save data.

D. Use kv.store(["key"], value) to save data.

Solution

1. Step 1: Understand Deno KV key format

   Deno KV uses arrays as keys, so keys must be passed as arrays like ["key"].

2. Step 2: Identify the correct method to save data

   The method to save data is set, so kv.set(["key"], value) is correct.

3. Final Answer:

   Use kv.set(["key"], value) to save data. -> Option A

4. Quick Check:

   Key as array + set method = Use kv.set(["key"], value) to save data. [OK]

Hint: Remember: keys are arrays and method is set [OK]

Common Mistakes:

• Using string keys instead of array keys
• Using wrong method names like save or put
• Passing key as a single string instead of array

2. Which of the following is the correct syntax to read a value from Deno KV?

easy

A. const result = kv.get(["key"]);

B. const result = await kv.get("key");

C. const result = await kv.get(["key"]);

D. const result = kv.get("key");

Solution

1. Step 1: Recognize key format for get method

   Deno KV requires keys as arrays, so the key must be passed as ["key"].

2. Step 2: Confirm asynchronous usage of get

   The get method is asynchronous, so it must be awaited.

3. Final Answer:

   const result = await kv.get(["key"]); -> Option C

4. Quick Check:

   Await + array key + get method = const result = await kv.get(["key"]); [OK]

Hint: Always await get and use array keys [OK]

Common Mistakes:

• Not awaiting the get method
• Passing key as string instead of array
• Using synchronous call without await

3. What will be the output of this code snippet?

const kv = await Deno.openKv();
await kv.set(["user", "name"], "Alice");
const result = await kv.get(["user", "name"]);
console.log(result.value);

medium

A. undefined

B. "Alice"

C. null

D. Throws an error

Solution

1. Step 1: Store value with key ["user", "name"]

   The code stores the string "Alice" under the key array ["user", "name"].

2. Step 2: Retrieve the stored value using the same key

   The get method fetches the stored value, which is "Alice".

3. Final Answer:

   "Alice" -> Option B

4. Quick Check:

   Set then get with same key returns stored value [OK]

Hint: Set then get with same key returns stored value [OK]

Common Mistakes:

• Expecting key as string instead of array
• Not awaiting async calls
• Confusing result object with direct value

4. Identify the error in this Deno KV usage:

const kv = await Deno.openKv();
await kv.set("session", "active");
const status = await kv.get(["session"]);
console.log(status.value);

medium

A. The key in set should be an array, not a string.

B. The get method should not be awaited.

C. The value "active" must be an object, not a string.

D. The console.log should print status directly, not status.value.

Solution

1. Step 1: Check key format in set method

   The set method requires the key to be an array, but here it is a string "session".

2. Step 2: Confirm correct key format for get method

   The get method uses the key as an array ["session"], which is correct.

3. Final Answer:

   The key in set should be an array, not a string. -> Option A

4. Quick Check:

   Key must be array in set and get [OK]

Hint: Keys must always be arrays in set and get [OK]

Common Mistakes:

• Using string keys instead of array keys
• Not awaiting async methods
• Misunderstanding value access from get result

5. You want to store user preferences with keys like ["user", userId, "prefs"] and values as objects. Which code correctly updates preferences for userId 42 without overwriting other keys?

const kv = await Deno.openKv();
const userId = 42;
const newPrefs = { theme: "dark" };
// Which code below is correct?

hard

A. await kv.set([userId, "prefs"], newPrefs);

B. await kv.set(["user", "prefs"], newPrefs);

C. await kv.set(["user", userId], newPrefs);

D. await kv.set(["user", userId, "prefs"], newPrefs);

Solution

1. Step 1: Understand key structure for user preferences

   The key must include "user", the userId, and "prefs" to uniquely identify preferences per user.

2. Step 2: Check which option uses the full key correctly

   await kv.set(["user", userId, "prefs"], newPrefs); uses ["user", userId, "prefs"] which matches the required key structure.

3. Final Answer:

   await kv.set(["user", userId, "prefs"], newPrefs); -> Option D

4. Quick Check:

   Full key array with userId keeps keys unique [OK]

Hint: Include userId in key array to avoid overwriting [OK]

Common Mistakes:

• Omitting userId from key causing overwrites
• Using incomplete key arrays
• Confusing order of key elements