KV on Deno Deploy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Deno KV code snippet?

Consider this code running on Deno Deploy with KV enabled:

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?.name);

What will be printed in 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?.name);

A null

B undefined

C Alice

D Throws a runtime error

💡 Hint

Think about what kv.get returns and how to access the stored value.

The kv.get method returns an object with a value property containing the stored data. Accessing result.value?.name safely retrieves the name Alice.

📝 Syntax

intermediate

Which option correctly deletes a key from Deno KV?

You want to delete the key ["session", "abc"] from Deno KV. Which code snippet will do this correctly?

A await kv.deleteKey(["session", "abc"]);

B await kv.delete(["session", "abc"]);

C await kv.del(["session", "abc"]);

D await kv.remove(["session", "abc"]);

💡 Hint

Check the official Deno KV API method names.

The correct method to delete a key is delete. Other options are invalid method names and will cause errors.

❓ state_output

advanced

What is the value of count after this transaction?

Given this code snippet using a Deno KV transaction:

const kv = await Deno.openKv();
await kv.set(["counter"], 0);

const tx = kv.atomic();
const current = await tx.get(["counter"]);
await tx.set(["counter"], (current.value ?? 0) + 1);
await tx.commit();

const result = await kv.get(["counter"]);
const count = result.value;

What is the value of count after running this code once?

Deno

const kv = await Deno.openKv();
await kv.set(["counter"], 0);

const tx = kv.atomic();
const current = await tx.get(["counter"]);
await tx.set(["counter"], (current.value ?? 0) + 1);
await tx.commit();

const result = await kv.get(["counter"]);
const count = result.value;

A 1

B undefined

C 0

D Throws a runtime error

💡 Hint

Think about how the transaction increments the counter.

The transaction reads the current value (0), adds 1, sets it, and commits. So the final value is 1.

🔧 Debug

advanced

Why does this Deno KV code throw an error?

Look at this code snippet:

const kv = await Deno.openKv();
await kv.set("user:1", { name: "Bob" });

It throws a runtime error. What is the cause?

Deno

const kv = await Deno.openKv();
await kv.set("user:1", { name: "Bob" });

A The key must be an array of strings or numbers, not a single string

B The value must be a string, not an object

C Deno KV requires keys to be numbers only

D The set method requires a third argument for options

💡 Hint

Check the expected key format for Deno KV.

Deno KV keys must be tuples (arrays) of strings or numbers. Passing a single string causes a type error.

🧠 Conceptual

expert

Which statement about Deno KV consistency is true?

Consider Deno KV's consistency guarantees. Which of these statements is correct?

A Deno KV provides strong consistency for all reads and writes globally

B Deno KV provides eventual consistency for reads but strong consistency for writes within a transaction

C Deno KV does not support transactions and consistency is not guaranteed

D Deno KV provides strong consistency only within atomic transactions, but reads outside may be eventually consistent

💡 Hint

Think about how atomic transactions affect consistency in distributed KV stores.

Deno KV guarantees strong consistency within atomic transactions. Outside transactions, reads may be eventually consistent due to distributed nature.

Practice

1. What is the main purpose of KV on Deno Deploy?

easy

A. To create user interfaces

B. To run heavy computations on the server

C. To store small pieces of data in the cloud

D. To manage network connections

Solution

1. Step 1: Understand KV storage role

   KV on Deno Deploy is designed to store small data pieces like counters or preferences.

2. Step 2: Compare options with KV purpose

   Only To store small pieces of data in the cloud matches storing small data in the cloud, others describe unrelated tasks.

3. Final Answer:

   To store small pieces of data in the cloud -> Option C

4. Quick Check:

   KV stores small data = A [OK]

Hint: KV is for small cloud data storage, not heavy tasks [OK]

Common Mistakes:

• Confusing KV with computation services
• Thinking KV manages UI or network
• Assuming KV handles large files

2. Which of the following is the correct syntax to save a value '42' with key 'count' using KV in Deno Deploy?

easy

A. await kv.set('count', 42);

B. kv.set('count', 42);

C. await kv.get('count', 42);

D. kv.get('count', 42);

Solution

1. Step 1: Identify correct method to save data

   Saving data uses kv.set with await to handle the async operation.

2. Step 2: Check syntax correctness

   await kv.set('count', 42); uses await kv.set('count', 42); which is correct syntax. Options C and D use get which reads data, not saves.

3. Final Answer:

   await kv.set('count', 42); -> Option A

4. Quick Check:

   Save data = await kv.set() [OK]

Hint: Use await with kv.set to save data asynchronously [OK]

Common Mistakes:

• Omitting await causing unresolved promises
• Using kv.get instead of kv.set to save
• Passing wrong number of arguments

3. What will be the output of this code snippet on Deno Deploy KV?

const value = await kv.get('visits');
console.log(value);

Assuming 'visits' key was never set before.

medium

A. undefined

B. null

C. Throws an error

D. 0

Solution

1. Step 1: Understand kv.get behavior for missing keys

   If a key does not exist, kv.get returns null, not undefined or error.

2. Step 2: Analyze console.log output

   Since 'visits' was never set, value is null, so console.log prints null.

3. Final Answer:

   null -> Option B

4. Quick Check:

   Missing key returns null = C [OK]

Hint: Missing KV keys return null, not undefined or error [OK]

Common Mistakes:

• Expecting undefined instead of null
• Assuming kv.get throws error for missing keys
• Thinking default numeric 0 is returned

4. Identify the error in this Deno Deploy KV code snippet:

const result = kv.set('score', 100);
console.log('Score saved:', result);

medium

A. Missing await before kv.set causing unresolved promise

B. kv.set does not accept string keys

C. kv.set should be kv.save

D. console.log syntax is incorrect

Solution

1. Step 1: Check async usage of kv.set

   kv.set is asynchronous and returns a promise, so it needs await to resolve.

2. Step 2: Verify other parts of code

   Keys can be strings, kv.set is correct method, and console.log syntax is valid.

3. Final Answer:

   Missing await before kv.set causing unresolved promise -> Option A

4. Quick Check:

   Async kv.set needs await = D [OK]

Hint: Always await kv.set to avoid unresolved promises [OK]

Common Mistakes:

• Forgetting await on async kv.set
• Using wrong method name like kv.save
• Misunderstanding console.log syntax

5. You want to implement a visitor counter using KV on Deno Deploy. Which approach correctly increments the count each time a visitor arrives?

hard

A. await kv.set('visits', (await kv.get('visits')) + 1);

B. kv.set('visits', 1);

C. const current = kv.get('visits') || 0;
kv.set('visits', current + 1);

D. const current = await kv.get('visits') ?? 0;
await kv.set('visits', current + 1);

Solution

1. Step 1: Retrieve current count safely

   Use await kv.get('visits') and default to 0 if null using ?? operator.

2. Step 2: Increment and save updated count

   Add 1 to current count and await kv.set to save it asynchronously.

3. Step 3: Analyze options for correctness

   const current = await kv.get('visits') ?? 0; await kv.set('visits', current + 1); correctly handles async and null values. await kv.set('visits', (await kv.get('visits')) + 1); misses null check, C misses await, D resets count always.

4. Final Answer:

   const current = await kv.get('visits') ?? 0; await kv.set('visits', current + 1); -> Option D

5. Quick Check:

   Safe get + increment + await set = B [OK]

Hint: Use null coalescing and await both get and set [OK]

Common Mistakes:

• Not awaiting async calls
• Not handling null from kv.get
• Resetting count instead of incrementing