Deno KV (built-in key-value store) - Mini Project: Build & Apply

Simple To-Do List with Deno KV

📖 Scenario: You want to create a simple to-do list app that saves tasks using Deno's built-in key-value store called Deno KV. This app will let you add tasks and mark them as done, storing all data persistently.

🎯 Goal: Build a small Deno script that stores to-do tasks in Deno KV, adds new tasks, and retrieves all tasks to show their status.

📋 What You'll Learn

Create a Deno KV instance

Set a configuration variable for the task prefix key

Add a new task to the KV store with a unique key

Retrieve and list all tasks stored in KV

💡 Why This Matters

🌍 Real World

Deno KV is useful for small to medium apps needing fast, simple persistent storage without external databases.

💼 Career

Understanding Deno KV helps backend developers build lightweight services and APIs with built-in storage, improving efficiency and reducing dependencies.

1

Create a Deno KV instance

Write a line of code to create a Deno KV instance and assign it to a variable called kv using await Deno.openKv().

Deno

# Create a Deno KV instance
# Your code here

Hint

Use await Deno.openKv() to create the KV instance and assign it to kv.

2

Set a task key prefix

Create a constant variable called TASK_PREFIX and set it to the string "task" to use as a prefix for task keys.

Deno

const kv = await Deno.openKv();
# Create a constant TASK_PREFIX with value "task"
# Your code here

Hint

Use const TASK_PREFIX = "task"; to define the prefix.

3

Add a new task to KV

Write an async function called addTask that takes a taskId and description. Inside, use kv.set to store the task with key [TASK_PREFIX, taskId] and value as an object with description and done: false.

Deno

const kv = await Deno.openKv();
const TASK_PREFIX = "task";

// Write async function addTask(taskId, description) to store task
# Your code here

Hint

Define addTask as async and use await kv.set([TASK_PREFIX, taskId], { description, done: false }).

4

Retrieve and list all tasks

Write an async function called listTasks that uses kv.list with prefix [TASK_PREFIX] to get all tasks. Use a for await loop to iterate over entries and log each task's taskId and description.

Deno

const kv = await Deno.openKv();
const TASK_PREFIX = "task";

async function addTask(taskId, description) {
  await kv.set([TASK_PREFIX, taskId], { description, done: false });
}

// Write async function listTasks() to list all tasks
# Your code here

Hint

Use for await to loop over kv.list({ prefix: [TASK_PREFIX] }) and extract taskId and description from each entry.

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