Sequential vs parallel async execution in Node.js - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run two async functions one after another (sequentially).

Node.js

async function runSequential() {
  await fetchData1();
  await [1]();
}

Drag options to blanks, or click blank then click option'

AfetchData2

BfetchData3

CfetchData1

DfetchData4

Attempts:

3 left

💡 Hint

Common Mistakes

Calling both functions without await causes parallel execution.

Forgetting to await the second function.

To run async functions sequentially, you await each one in order. Here, after fetchData1(), you await fetchData2().

2fill in blank

medium

Complete the code to run two async functions in parallel and wait for both to finish.

Node.js

async function runParallel() {
  await Promise.[1]([fetchData1(), fetchData2()]);
}

Drag options to blanks, or click blank then click option'

Aany

BallSettled

Crace

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.race waits only for the first promise to settle.

Using Promise.any waits for the first fulfilled promise, not all.

Promise.all runs multiple promises in parallel and waits for all to resolve.

3fill in blank

hard

Fix the error in the code to correctly run async functions sequentially.

Node.js

async function run() {
  await fetchData1();
  [1] fetchData2();
}

Drag options to blanks, or click blank then click option'

Athen

Basync

Cawait

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Calling async functions without await runs them in parallel.

Using async keyword incorrectly inside the function body.

To run async functions sequentially, you must await each call to pause until it finishes.

4fill in blank

hard

Fill both blanks to create a parallel async execution that handles errors gracefully.

Node.js

async function runSafeParallel() {
  const results = await Promise.[1]([
    fetchData1(),
    fetchData2()
  ]);
  results.forEach(result => {
    if (result.status === '[2]') {
      console.log('Success:', result.value);
    } else {
      console.error('Error:', result.reason);
    }
  });
}

Drag options to blanks, or click blank then click option'

AallSettled

Bfulfilled

Crejected

Dresolved

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.all rejects immediately on any error.

Checking for status 'resolved' instead of 'fulfilled'.

Promise.allSettled waits for all promises and returns their status. Checking for fulfilled lets you handle successes and errors separately.

5fill in blank

hard

Fill all three blanks to create a dictionary of results from parallel async calls filtering only successful ones.

Node.js

async function getResults() {
  const results = await Promise.[1]([
    fetchData1(),
    fetchData2(),
    fetchData3()
  ]);
  return results.reduce((acc, [2], index) => {
    if ([3].status === 'fulfilled') {
      acc[`data${index + 1}`] = [2].value;
    }
    return acc;
  }, {});
}

Drag options to blanks, or click blank then click option'

AallSettled

Bresult

Cresults

Dres

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.all which rejects on first error.

Using wrong variable names causing runtime errors.

Promise.allSettled returns an array of result objects. Using result as the variable name for each item, you check if result.status is 'fulfilled' to include only successful results.

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