Deno KV (built-in key-value store) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Deno KV get operation?

Consider this code snippet using Deno KV:

const kv = await Deno.openKv();
await kv.set(["user", "123"], { name: "Alice", age: 30 });
const result = await kv.get(["user", "123"]);
console.log(result.value);

What will be logged to the console?

Deno

const kv = await Deno.openKv();
await kv.set(["user", "123"], { name: "Alice", age: 30 });
const result = await kv.get(["user", "123"]);
console.log(result.value);

A null

B undefined

C {"name":"Alice","age":30}

D {"user":"123"}

💡 Hint

Remember that kv.get() returns an object with a value property containing the stored data.

The kv.get() method returns an object with a value property holding the stored value. Since we stored { name: "Alice", age: 30 } under the key ["user", "123"], result.value will be that object.

❓ state_output

intermediate

What is the value of 'count' after these Deno KV operations?

Given this code:

const kv = await Deno.openKv();
await kv.set(["counter"], 1);
await kv.set(["counter"], 2);
const { value: count } = await kv.get(["counter"]);
console.log(count);

What will be printed?

Deno

const kv = await Deno.openKv();
await kv.set(["counter"], 1);
await kv.set(["counter"], 2);
const { value: count } = await kv.get(["counter"]);
console.log(count);

A 2

B 1

C undefined

D null

💡 Hint

Think about what happens when you set the same key twice.

Setting the same key twice overwrites the previous value. The last set value is 2, so count will be 2.

🔧 Debug

advanced

Which option will cause a runtime error when using Deno KV keys?

Deno KV keys must be arrays of strings or numbers. Which of these key usages will cause a runtime error?

Deno

const kv = await Deno.openKv();
await kv.set(KEY, "value");

A KEY = ["user", 123];

B KEY = "user:123";

C KEY = ["user", "123"];

D KEY = ["user", 123, "profile"];

💡 Hint

Remember that keys must be arrays, not strings.

Deno KV keys must be arrays of strings or numbers. Using a plain string like "user:123" as a key is invalid and will cause a runtime error.

🔧 Debug

advanced

Why does this Deno KV code throw a TypeError?

Look at this code:

const kv = await Deno.openKv();
await kv.set("user", { name: "Bob" });
const result = await kv.get("user");
console.log(result.value);

Why does it throw a TypeError?

Deno

const kv = await Deno.openKv();
await kv.set("user", { name: "Bob" });
const result = await kv.get("user");
console.log(result.value);

A Because keys must be arrays, not strings

B Because the value must be a string, not an object

C Because Deno KV does not support await

D Because the key "user" is reserved

💡 Hint

Check the type of the key argument in set and get.

Deno KV requires keys to be arrays of strings or numbers. Passing a string directly causes a TypeError.

🧠 Conceptual

expert

Which option correctly describes Deno KV transactions?

Which statement about Deno KV transactions is true?

A Deno KV does not support transactions; each operation is independent

B Transactions in Deno KV are synchronous and block the event loop until complete

C Transactions require manual locking of keys before operations

D Transactions allow multiple operations to be executed atomically and can be retried automatically on conflicts

💡 Hint

Think about atomicity and concurrency control in key-value stores.

Deno KV supports transactions that group multiple operations atomically. They can be retried automatically if conflicts occur, ensuring consistency.

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