Sequential vs parallel execution in Javascript - Performance Comparison

Time Complexity: Sequential vs parallel execution

O(n)

Understanding Time Complexity

When running tasks one after another or at the same time, the total time changes. We want to see how the time grows as we add more tasks.

How does doing things in order compare to doing them together?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Sequential execution
async function runSequential(tasks) {
  for (const task of tasks) {
    await task();
  }
}

// Parallel execution
async function runParallel(tasks) {
  await Promise.all(tasks.map(task => task()));
}
    

This code runs a list of tasks either one by one or all at once.

Identify Repeating Operations

• Primary operation: Calling each task function once.
• How many times: Exactly once per task, total of n tasks.
• Sequential uses a loop with await inside, waiting for each task before next.
• Parallel starts all tasks at once, waiting for all to finish together.

How Execution Grows With Input

Explain the growth pattern intuitively.

Input Size (n)	Approx. Operations
10	10 tasks run one after another or all at once
100	100 tasks run one after another or all at once
1000	1000 tasks run one after another or all at once

In sequential, total time adds up for each task. In parallel, total time is about the longest single task time, not the sum.

Final Time Complexity

Time Complexity: O(n)

This means the total time grows linearly with the number of tasks when running sequentially. Parallel execution can be faster but still involves starting all tasks once.

Common Mistake

[X] Wrong: "Running tasks in parallel always takes the same time as one task."

[OK] Correct: Even in parallel, the total time depends on the longest task, so if one task is slow, it slows down the whole batch.

Interview Connect

Understanding how tasks run one by one or together helps you explain real-world code performance. It shows you can think about how programs handle many jobs efficiently.

Self-Check

"What if tasks had different lengths and we ran them in batches of 5 instead of all at once? How would the time complexity change?"

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