Sequential vs parallel async execution in Node.js - When to Use Which

The Big Idea

What if your app could do many things at once and finish tasks way faster?

The Scenario

Imagine you need to fetch data from three websites one after another, waiting for each to finish before starting the next.

The Problem

Doing this one by one makes your app slow and unresponsive because it wastes time waiting for each task to finish before starting the next.

The Solution

Using parallel async execution, you can start all tasks at once and wait for all to finish together, making your app faster and more efficient.

Before vs After

✗ Before

const data1 = await fetch(url1);
const data2 = await fetch(url2);
const data3 = await fetch(url3);

✓ After

const [data1, data2, data3] = await Promise.all([fetch(url1), fetch(url2), fetch(url3)]);

What It Enables

This lets your app do many things at the same time, saving time and improving user experience.

Real Life Example

Loading images on a webpage simultaneously instead of one by one so the page appears faster to users.

Key Takeaways

Sequential execution waits for each task before starting the next.

Parallel execution runs tasks together, saving time.

Using parallel async makes apps faster and smoother.

Practice

1. What is the main difference between sequential and parallel async execution in Node.js?

easy

A. Sequential async is faster than parallel async in all cases.

B. Sequential async waits for each task to finish before starting the next, while parallel async runs tasks at the same time.

C. Sequential async uses callbacks, while parallel async uses promises.

D. Sequential async runs all tasks at once, while parallel async runs them one by one.

Solution

1. Step 1: Understand sequential async execution

   Sequential async means tasks run one after another, waiting for each to complete before starting the next.

2. Step 2: Understand parallel async execution

   Parallel async means tasks run at the same time, without waiting for others to finish first.

3. Final Answer:

   Sequential async waits for each task to finish before starting the next, while parallel async runs tasks at the same time. -> Option B

4. Quick Check:

   Sequential vs parallel async = D [OK]

Hint: Sequential waits, parallel runs together [OK]

Common Mistakes:

• Confusing which runs tasks one by one
• Thinking parallel always uses callbacks
• Assuming sequential is always faster

2. Which of the following is the correct syntax to run two async functions task1() and task2() in parallel using Promise.all?

easy

A. await Promise.all([task1(), task2()]);

B. Promise.all(task1(), task2());

C. await task1(); await task2();

D. task1().then(task2());

Solution

1. Step 1: Recall Promise.all syntax

   Promise.all takes an array of promises and waits for all to complete in parallel.

2. Step 2: Check correct usage

   The correct syntax is await Promise.all([task1(), task2()]) to run both tasks in parallel and wait for both.

3. Final Answer:

   await Promise.all([task1(), task2()]); -> Option A

4. Quick Check:

   Promise.all needs array of promises [OK]

Hint: Use Promise.all with array for parallel [OK]

Common Mistakes:

• Missing array brackets in Promise.all
• Awaiting tasks one by one (sequential)
• Calling then() incorrectly without chaining

3. Consider this code snippet:

async function run() {
  const result1 = await task1();
  const result2 = await task2();
  return [result1, result2];
}
run().then(console.log);

What will be the order of execution and output behavior?

medium

A. task1 and task2 run in parallel; output is an array of both results.

B. Both tasks run sequentially but output is only result2.

C. task2 runs first, then task1; output is an array with reversed results.

D. task1 runs first, then task2 starts after task1 finishes; output is an array of both results.

Solution

1. Step 1: Analyze await usage

   Each await pauses execution until the promise resolves, so task1 finishes before task2 starts.

2. Step 2: Understand output array

   Both results are collected in order into an array and returned, so output is [result1, result2].

3. Final Answer:

   task1 runs first, then task2 starts after task1 finishes; output is an array of both results. -> Option D

4. Quick Check:

   Sequential await = B [OK]

Hint: Await pauses; tasks run one after another [OK]

Common Mistakes:

• Assuming tasks run in parallel with sequential await
• Thinking output order is reversed
• Believing output contains only one result

4. Identify the error in this code snippet intended to run two async tasks in parallel:

async function run() {
  const results = await Promise.all(task1(), task2());
  console.log(results);
}

medium

A. Promise.all requires an array of promises, but here arguments are passed separately.

B. Await cannot be used with Promise.all.

C. task1 and task2 must be awaited separately before Promise.all.

D. console.log cannot print arrays.

Solution

1. Step 1: Check Promise.all argument

   Promise.all expects a single array of promises, but here two arguments are passed separately.

2. Step 2: Correct usage

   It should be Promise.all([task1(), task2()]) with square brackets to group promises.

3. Final Answer:

   Promise.all requires an array of promises, but here arguments are passed separately. -> Option A

4. Quick Check:

   Promise.all needs array argument [OK]

Hint: Promise.all needs array, not separate args [OK]

Common Mistakes:

• Passing promises as separate arguments
• Thinking await can't be used with Promise.all
• Misunderstanding console.log capabilities

5. You have three independent async tasks: taskA(), taskB(), and taskC(). You want to run taskA and taskB in parallel, then run taskC only after both finish. Which code correctly implements this?

hard

A. const c = await taskC();
const [a, b] = await Promise.all([taskA(), taskB()]);

B. await taskA(); await taskB(); await taskC();

C. const [a, b] = await Promise.all([taskA(), taskB()]);
const c = await taskC();

D. Promise.all([taskA(), taskB(), taskC()]);

Solution

1. Step 1: Run taskA and taskB in parallel

   Using await Promise.all([taskA(), taskB()]) runs both tasks at the same time and waits for both to finish.

2. Step 2: Run taskC after both finish

   After awaiting both, await taskC() runs taskC sequentially, ensuring it starts only after taskA and taskB complete.

3. Final Answer:

   const [a, b] = await Promise.all([taskA(), taskB()]); const c = await taskC(); -> Option C

4. Quick Check:

   Parallel first, then sequential = A [OK]

Hint: Use Promise.all for parallel, then await next task [OK]

Common Mistakes:

• Running all tasks in parallel ignoring order
• Running tasks sequentially without parallelism
• Starting taskC before taskA and taskB finish