Sequential vs parallel async execution in Node.js - Quick Revision & Key Differences

Recall & Review

beginner

What does sequential async execution mean in Node.js?

Sequential async execution means running asynchronous tasks one after another, waiting for each to finish before starting the next.

beginner

What is parallel async execution?

Parallel async execution means starting multiple asynchronous tasks at the same time and waiting for all to finish together.

beginner

Which Node.js function helps run async tasks in parallel and wait for all to complete?

Promise.all() runs multiple promises in parallel and waits for all of them to resolve or any to reject.

intermediate

Why might you choose sequential async execution over parallel?

You choose sequential execution when tasks depend on each other or when you want to limit resource use to avoid overload.

beginner

What is a downside of running async tasks sequentially?

Sequential execution can be slower because each task waits for the previous one to finish before starting.

Which method runs async tasks in parallel in Node.js?

A Promise.all()

B setTimeout()

C async function() {}

D console.log()

Promise.all() runs multiple promises at the same time and waits for all to finish.

What happens in sequential async execution?

A Tasks run randomly

B Tasks run all at once without waiting

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

D Tasks run only if previous failed

Sequential means tasks wait for the previous to finish before starting.

Which is a benefit of parallel async execution?

A Faster overall completion

B Uses less memory

C Runs tasks one by one

D Avoids all errors

Running tasks in parallel can finish faster since they happen at the same time.

When should you prefer sequential async execution?

A When you want to run tasks randomly

B When you want fastest speed

C When tasks are independent

D When tasks depend on each other

Sequential is best when one task needs the result of the previous.

What does Promise.all() return?

A An array of functions

B A promise that resolves when all input promises resolve

C A single value immediately

D A callback function

Promise.all() returns a promise that resolves with all results once all promises finish.

Explain the difference between sequential and parallel async execution in Node.js.

Think about waiting times and task order.

When would you choose sequential async execution over parallel, and why?

Consider task dependencies and resource limits.

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