Sequential vs parallel execution in Javascript - Hands-On Comparison

Sequential vs Parallel Execution in JavaScript

📖 Scenario: Imagine you are cooking breakfast. You can either cook each item one after another (sequentially) or cook some items at the same time (in parallel) to save time.

🎯 Goal: You will write JavaScript code to see how sequential and parallel execution works using simple timers. You will create two functions: one that runs tasks one after another, and one that runs tasks at the same time.

📋 What You'll Learn

Create an array of tasks with names and durations

Create a function to run tasks sequentially using async/await

Create a function to run tasks in parallel using Promise.all

Print the order and timing of task completions

💡 Why This Matters

🌍 Real World

Understanding sequential and parallel execution helps you write faster and more efficient programs, like loading images or data on websites.

💼 Career

Many programming jobs require knowledge of asynchronous code and how to manage tasks that take time, such as network requests or file operations.

1

Create the tasks array

Create an array called tasks with these exact objects: {name: 'Toast', duration: 1000}, {name: 'Eggs', duration: 2000}, and {name: 'Coffee', duration: 1500}.

Javascript

// Create the tasks array with the given objects
// Your code here

Hint

Use const tasks = [ ... ] with objects inside curly braces.

2

Create a helper function to simulate a task

Create an async function called runTask that takes a task object and returns a Promise that resolves after task.duration milliseconds with the message `Finished ${task.name}`.

Javascript

const tasks = [
  {name: 'Toast', duration: 1000},
  {name: 'Eggs', duration: 2000},
  {name: 'Coffee', duration: 1500}
];

// Create async function runTask(task) that returns a Promise resolving after task.duration
// Your code here

Hint

Use setTimeout inside a Promise to wait for task.duration milliseconds.

3

Create a function to run tasks sequentially

Create an async function called runSequential that loops over tasks using for (const task of tasks), awaits runTask(task), and logs the result with console.log.

Javascript

const tasks = [
  {name: 'Toast', duration: 1000},
  {name: 'Eggs', duration: 2000},
  {name: 'Coffee', duration: 1500}
];

async function runTask(task) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(`Finished ${task.name}`);
    }, task.duration);
  });
}

// Create async function runSequential() that loops over tasks and logs each finished message
// Your code here

Hint

Use a for...of loop and await inside the loop to run tasks one by one.

4

Run tasks sequentially and in parallel, then print results

Add code to call runSequential() and wait for it to finish. Then create an async function runParallel that uses Promise.all on tasks.map(task => runTask(task)) and logs each result. Finally, call runParallel().

Javascript

const tasks = [
  {name: 'Toast', duration: 1000},
  {name: 'Eggs', duration: 2000},
  {name: 'Coffee', duration: 1500}
];

async function runTask(task) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(`Finished ${task.name}`);
    }, task.duration);
  });
}

async function runSequential() {
  for (const task of tasks) {
    const result = await runTask(task);
    console.log(result);
  }
}

// Call runSequential(), then create async function runParallel() using Promise.all and log results
// Call runParallel() after
// Your code here

Hint

Use an async IIFE to run runSequential() then runParallel(). Use Promise.all with tasks.map for parallel execution.

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