Sequential vs parallel execution in Javascript

Introduction

Sequential and parallel execution help us control how tasks run in a program. Sequential runs tasks one after another, while parallel runs tasks at the same time.

When you want to do steps in order, like making a sandwich step-by-step.

When you want to do many things at once, like cooking multiple dishes at the same time.

When waiting for slow tasks like loading images or data from the internet.

When you want your program to be faster by doing tasks together.

When you want to keep your app responsive while doing background work.

Syntax

Javascript

// Sequential example
console.log('Start');
console.log('Middle');
console.log('End');

// Parallel example with promises
Promise.all([
  fetch('url1'),
  fetch('url2')
]).then(results => {
  console.log('Both done');
});

Sequential code runs line by line, waiting for each to finish.

Parallel code can run tasks at the same time, often using promises or async functions.

Examples

This runs three messages one after another, in order.

Javascript

console.log('First');
console.log('Second');
console.log('Third');

This runs two tasks at the same time and prints results when both finish.

Javascript

Promise.all([
  new Promise(res => setTimeout(() => res('A done'), 1000)),
  new Promise(res => setTimeout(() => res('B done'), 500))
]).then(results => console.log(results));

Sample Program

This program first runs three steps one after another with 1 second wait each. Then it runs three steps at the same time, all finishing after about 1 second.

Javascript

console.log('Sequential start');

function wait(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

async function runSequential() {
  console.log('Step 1');
  await wait(1000);
  console.log('Step 2');
  await wait(1000);
  console.log('Step 3');
}

async function runParallel() {
  const p1 = wait(1000).then(() => console.log('Parallel Step 1'));
  const p2 = wait(1000).then(() => console.log('Parallel Step 2'));
  const p3 = wait(1000).then(() => console.log('Parallel Step 3'));
  await Promise.all([p1, p2, p3]);
}

(async () => {
  await runSequential();
  console.log('Sequential done');
  await runParallel();
  console.log('Parallel done');
})();

Important Notes

Sequential code is easier to understand but can be slower if tasks wait for each other.

Parallel code can be faster but needs care to handle results and errors.

JavaScript uses promises and async/await to help write parallel code clearly.

Summary

Sequential execution runs tasks one by one in order.

Parallel execution runs tasks at the same time to save time.

Use async/await and promises in JavaScript to manage parallel tasks easily.

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