Sequential vs parallel execution in Javascript - Quick Revision & Key Differences

Recall & Review

beginner

What is sequential execution in JavaScript?

Sequential execution means running code one step at a time, in order. Each step waits for the previous one to finish before starting.

beginner

What does parallel execution mean?

Parallel execution means running multiple tasks at the same time, so they can finish faster by working together.

intermediate

How does JavaScript handle parallel execution since it has a single thread?

JavaScript uses asynchronous features like promises and async/await to start tasks that run in the background, allowing other code to run without waiting.

intermediate

What is the role of Promise.all() in parallel execution?

Promise.all() lets you run multiple promises at the same time and wait for all of them to finish before continuing.

intermediate

Give an example of sequential vs parallel execution using async functions.

Sequential: await task1(); await task2(); runs task2 after task1 finishes.
Parallel: await Promise.all([task1(), task2()]); runs both tasks at the same time.

Which of these best describes sequential execution?

A Tasks run randomly

B Tasks run all at the same time

C Tasks run one after another, waiting for each to finish

D Tasks never finish

Sequential execution means tasks run one by one, each waiting for the previous to finish.

What JavaScript feature helps run tasks in parallel?

A Console.log

B For loops

C Variables

D Promises and async/await

Promises and async/await allow JavaScript to handle tasks asynchronously, enabling parallel execution.

What does Promise.all() do?

A Runs multiple promises in parallel and waits for all to finish

B Runs promises one by one

C Cancels all promises

D Creates a new promise that never resolves

Promise.all() runs promises together and waits until all complete.

If you write await task1(); await task2();, how do tasks run?

A In parallel, both start together

B Sequentially, task2 waits for task1

C Only task2 runs

D Both tasks are skipped

Using await one after another runs tasks sequentially.

Why is parallel execution useful?

A It can make programs faster by doing many things at once

B It makes code harder to read

C It always uses more memory

D It stops programs from running

Parallel execution helps programs finish tasks faster by running them simultaneously.

Explain the difference between sequential and parallel execution in JavaScript.

Think about waiting for tasks to finish or running them together.

How does Promise.all() help with parallel execution?

It collects many promises and waits for all.

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