Sequential vs parallel execution in Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run two functions one after the other (sequentially).

Javascript

function first() {
  console.log('First');
}

function second() {
  console.log('Second');
}

first();
[1]();

Drag options to blanks, or click blank then click option'

Asecond

BsetTimeout

CPromise

Dasync

Attempts:

3 left

💡 Hint

Common Mistakes

Using setTimeout or Promise here causes asynchronous behavior, not sequential direct calls.

Forgetting to add parentheses to call the function.

Calling second() after first() runs the functions one after the other, which is sequential execution.

2fill in blank

medium

Complete the code to run two asynchronous functions in parallel using Promise.all.

Javascript

function task1() {
  return new Promise(resolve => setTimeout(() => resolve('Task 1 done'), 1000));
}

function task2() {
  return new Promise(resolve => setTimeout(() => resolve('Task 2 done'), 500));
}

Promise.[1]([task1(), task2()]).then(results => {
  console.log(results);
});

Drag options to blanks, or click blank then click option'

Arace

Bcatch

Cthen

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.race runs only the first promise that finishes.

Using then or catch directly on Promise does not run multiple promises.

Promise.all runs multiple promises in parallel and waits for all to finish before continuing.

3fill in blank

hard

Fix the error in the async function to run tasks sequentially using await.

Javascript

async function runTasks() {
  const result1 = task1();
  const result2 = task2();
  console.log(await result1);
  console.log(await result2);
}

runTasks();

// Fix the line: const result1 = [1];

Drag options to blanks, or click blank then click option'

AsetTimeout(task1, 1000)

Bawait task1()

Ctask1()

DPromise.resolve(task1())

Attempts:

3 left

💡 Hint

Common Mistakes

Not using await causes the tasks to start but not wait for completion.

Using setTimeout does not return a promise and won't work with await.

Using await before task1() ensures the function waits for task1 to finish before continuing, enabling sequential execution.

4fill in blank

hard

Fill both blanks to create a function that runs two tasks in parallel and logs their results.

Javascript

async function runParallel() {
  const [result1, result2] = await Promise.[1]([task1(), task2()]);
  console.log(result1);
  console.log([2]);
}

runParallel();

Drag options to blanks, or click blank then click option'

Aall

Bresult2

Crace

Dresult1

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.race only waits for the first task to finish.

Logging result1 twice misses the second task's result.

Promise.all runs tasks in parallel. Logging result1 and result2 shows both results after completion.

5fill in blank

hard

Fill all three blanks to create a function that runs tasks sequentially using await and logs results.

Javascript

async function runSequential() {
  const result1 = await [1]();
  console.log(result1);
  const result2 = await [2]();
  console.log([3]);
}

runSequential();

Drag options to blanks, or click blank then click option'

Atask1

Btask2

Cresult2

Dresult1

Attempts:

3 left

💡 Hint

Common Mistakes

Calling tasks without await runs them in parallel.

Logging the wrong result variable causes confusion.

Awaiting task1 and task2 runs them one after the other. Logging result1 and result2 shows their outputs in order.

Practice

1.

What is the main difference between sequential and parallel execution in JavaScript?

easy

A. Sequential uses promises; parallel uses only callbacks.

B. Sequential runs tasks one after another; parallel runs tasks at the same time.

C. Sequential runs tasks at the same time; parallel runs tasks one after another.

D. Sequential is faster than parallel always.

Solution

1. Step 1: Understand sequential execution

   Sequential execution means tasks run one by one, waiting for each to finish before starting the next.

2. Step 2: Understand parallel execution

   Parallel execution means tasks run at the same time, so they can finish faster by overlapping.

3. Final Answer:

   Sequential runs tasks one after another; parallel runs tasks at the same time. -> Option B

4. Quick Check:

   Sequential = one by one, Parallel = at the same time [OK]

Hint: Remember: sequential = one by one, parallel = together [OK]

Common Mistakes:

• Confusing which runs tasks simultaneously
• Thinking sequential is always faster
• Mixing up async/await with execution type

2.

Which of the following is the correct syntax to run two promises in parallel using Promise.all?

const p1 = fetch(url1);
const p2 = fetch(url2);
// What is correct?

easy

A. const results = await Promise.race([p1, p2]);

B. const results = await Promise.parallel([p1, p2]);

C. const results = await Promise.sequence([p1, p2]);

D. const results = await Promise.all([p1, p2]);

Solution

1. Step 1: Identify correct Promise method for parallel

   Promise.all runs multiple promises in parallel and waits for all to finish.

2. Step 2: Check syntax correctness

   Using await Promise.all([p1, p2]) correctly waits for both promises in parallel.

3. Final Answer:

   const results = await Promise.all([p1, p2]); -> Option D

4. Quick Check:

   Promise.all = parallel wait [OK]

Hint: Use Promise.all for parallel waits, not race or sequence [OK]

Common Mistakes:

• Using Promise.race which waits only for first promise
• Using non-existent Promise.sequence or Promise.parallel
• Not wrapping promises in an array

3.

What will be the output of this code?

async function test() {
  console.log('Start');
  await new Promise(r => setTimeout(r, 1000));
  console.log('Middle');
  await new Promise(r => setTimeout(r, 1000));
  console.log('End');
}
test();

medium

A. Start immediately, then Middle after 1s, then End after 2s

B. Start, Middle, and End all print immediately

C. Start, End, then Middle after 2 seconds

D. Only Start prints, then program stops

Solution

1. Step 1: Analyze first await delay

   After printing 'Start', the code waits 1 second before printing 'Middle'.

2. Step 2: Analyze second await delay

   After 'Middle', it waits another 1 second before printing 'End'.

3. Final Answer:

   Start immediately, then Middle after 1s, then End after 2s -> Option A

4. Quick Check:

   await pauses sequentially, so output is delayed stepwise [OK]

Hint: await pauses code, so logs appear one after another [OK]

Common Mistakes:

• Thinking all logs print immediately
• Confusing setTimeout with immediate execution
• Ignoring await pauses

4.

Find the error in this code that tries to run two async tasks in parallel:

async function runTasks() {
  const result1 = await task1();
  const result2 = await task2();
  return [result1, result2];
}

medium

A. Cannot return array from async function.

B. Missing async keyword on task1 and task2.

C. Tasks run sequentially, not in parallel.

D. await cannot be used inside async functions.

Solution

1. Step 1: Check how tasks are awaited

   Each await waits for task1 to finish before starting task2, so tasks run one after another.

2. Step 2: Identify parallel execution fix

   To run in parallel, start both tasks without await, then await both together using Promise.all.

3. Final Answer:

   Tasks run sequentially, not in parallel. -> Option C

4. Quick Check:

   Separate awaits cause sequential execution [OK]

Hint: Awaiting one by one is sequential, start all then await all for parallel [OK]

Common Mistakes:

• Assuming await runs tasks in parallel
• Thinking async keyword fixes parallelism alone
• Believing async functions can't return arrays

5.

You want to fetch data from three URLs and process results only after all are done. Which code correctly runs fetches in parallel and waits for all?

const urls = ['url1', 'url2', 'url3'];
async function fetchAll() {
  // Fill this
}

hard

A. const results = await Promise.all(urls.map(url => fetch(url)));

B. const results = [];
for (const url of urls) {
results.push(await fetch(url));
}

C. const results = urls.map(url => await fetch(url));

D. const results = await urls.forEach(url => fetch(url));

Solution

1. Step 1: Understand parallel fetch with Promise.all

   Using Promise.all with map starts all fetches at once and waits for all to finish.

2. Step 2: Check other options for errors

   const results = []; for (const url of urls) { results.push(await fetch(url)); } fetches sequentially due to await inside loop; const results = urls.map(url => await fetch(url)); uses await incorrectly inside map; const results = await urls.forEach(url => fetch(url)); misuses forEach which returns undefined.

3. Final Answer:

   const results = await Promise.all(urls.map(url => fetch(url))); -> Option A

4. Quick Check:

   Promise.all + map = parallel fetch [OK]

Hint: Use Promise.all with map for parallel async calls [OK]

Common Mistakes:

• Await inside for loop causing sequential fetch
• Using await inside map callback directly
• Using forEach which does not return promises