Event loop phases and timer execution in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the event loop in Node.js?

The event loop is a process that handles asynchronous callbacks in Node.js. It allows Node.js to perform non-blocking operations by offloading tasks and executing callbacks when ready.

intermediate

Name the main phases of the Node.js event loop.

The main phases are: timers, pending callbacks, idle/prepare, poll, check, and close callbacks. Each phase has a specific role in processing callbacks and events.

beginner

What happens during the 'timers' phase of the event loop?

During the timers phase, callbacks scheduled by setTimeout() and setInterval() are executed if their time has expired.

intermediate

How does the 'poll' phase work in the event loop?

The poll phase waits for new I/O events and executes their callbacks. If there are no timers to run, it will wait for callbacks or new events to arrive.

intermediate

Why might a timer callback be delayed in Node.js?

Timer callbacks can be delayed if the event loop is busy running other callbacks or if the poll phase is waiting for I/O. Timers only run after their delay and when the event loop reaches the timers phase.

Which event loop phase executes callbacks from setTimeout()?

A check

B poll

C timers

D close callbacks

The timers phase runs callbacks scheduled by setTimeout() and setInterval().

What does the poll phase do in the Node.js event loop?

A Executes close callbacks

B Executes setImmediate callbacks

C Runs timer callbacks

D Waits for and processes I/O events

The poll phase waits for new I/O events and executes their callbacks.

When are setImmediate() callbacks executed?

A During the check phase

B During the poll phase

C During the timers phase

D During the close callbacks phase

setImmediate() callbacks run in the check phase, which happens after the poll phase.

If the event loop is busy, what happens to timer callbacks?

A They are delayed until the event loop is free

B They are skipped

C They run immediately

D They run during the poll phase

Timer callbacks wait until the event loop reaches the timers phase and is free to run them.

Which phase runs callbacks for socket or handle close events?

A timers

B close callbacks

C check

D poll

The close callbacks phase runs callbacks for closed sockets or handles.

Explain the sequence of event loop phases and how timer callbacks are executed in Node.js.

Think about how the event loop cycles through phases and when timers get their turn.

Describe why a setTimeout callback might not run exactly after its delay time in Node.js.

Consider what else the event loop might be doing when the timer expires.

Practice

1. Which phase of the Node.js event loop executes setTimeout callbacks?

easy

A. Check phase

B. Timers phase

C. Poll phase

D. Close callbacks phase

Solution

1. Step 1: Understand event loop phases

   The Node.js event loop has multiple phases, each handling different types of callbacks.

2. Step 2: Identify where timers run

   The timers phase is specifically designed to execute callbacks scheduled by setTimeout and setInterval.

3. Final Answer:

   Timers phase -> Option B

4. Quick Check:

   Timers phase = setTimeout callbacks [OK]

Hint: Timers run in the timers phase, not immediately [OK]

Common Mistakes:

• Confusing timers phase with poll phase
• Thinking setTimeout runs immediately
• Mixing check phase with timers phase

2. Which of the following is the correct syntax to schedule a function to run after 0 milliseconds in Node.js?

easy

A. setTimeout(console.log('Hi'))

B. setTimeout(console.log('Hi'), 0);

C. setTimeout(0, () => console.log('Hi'));

D. setTimeout(() => console.log('Hi'), 0);

Solution

1. Step 1: Check function syntax for setTimeout

   The first argument must be a function, not the result of a function call.

2. Step 2: Analyze each option

   setTimeout(() => console.log('Hi'), 0); passes a function that logs 'Hi' after 0 ms delay correctly. setTimeout(console.log('Hi'), 0); calls console.log immediately and passes its result (undefined). setTimeout(0, () => console.log('Hi')); passes 0 (number) as first argument instead of a function, causing a TypeError on execution. setTimeout(console.log('Hi')) calls console.log immediately without delay argument.

3. Final Answer:

   setTimeout(() => console.log('Hi'), 0); -> Option D

4. Quick Check:

   Function as first argument = setTimeout(() => console.log('Hi'), 0); [OK]

Hint: Pass a function, not a function call, to setTimeout [OK]

Common Mistakes:

• Calling the function immediately inside setTimeout
• Omitting the delay argument
• Passing non-function as first argument

3. What will be the output order of the following code?

console.log('Start');
setTimeout(() => console.log('Timeout 1'), 0);
setTimeout(() => console.log('Timeout 2'), 10);
console.log('End');

medium

A. Start, End, Timeout 1, Timeout 2

B. Start, Timeout 1, Timeout 2, End

C. Timeout 1, Timeout 2, Start, End

D. Start, Timeout 2, End, Timeout 1

Solution

1. Step 1: Identify synchronous and asynchronous parts

   console.log('Start') and console.log('End') run immediately in order. setTimeout callbacks run later.

2. Step 2: Understand timer delays and event loop

   setTimeout with 0 ms delay runs after current code finishes, before 10 ms delay callback.

3. Final Answer:

   Start, End, Timeout 1, Timeout 2 -> Option A

4. Quick Check:

   Synchronous logs first, then timers by delay [OK]

Hint: Synchronous logs run before any setTimeout callbacks [OK]

Common Mistakes:

• Assuming 0 ms timeout runs immediately
• Mixing order of synchronous and asynchronous logs
• Ignoring timer delays

4. Identify the error in this code snippet:

setTimeout(console.log('Hello'), 1000);

medium

A. The delay argument is missing

B. The delay must be 0 or less

C. console.log is called immediately instead of after 1000ms

D. setTimeout requires a string as first argument

Solution

1. Step 1: Analyze the first argument of setTimeout

   console.log('Hello') is called immediately, returning undefined, which is passed to setTimeout.

2. Step 2: Understand correct usage

   setTimeout expects a function as first argument, not the result of a function call.

3. Final Answer:

   console.log is called immediately instead of after 1000ms -> Option C

4. Quick Check:

   Function call inside setTimeout runs immediately [OK]

Hint: Pass a function, not a function call, to setTimeout [OK]

Common Mistakes:

• Thinking delay argument is missing
• Passing string instead of function
• Believing delay must be zero or negative

5. Consider this code:

console.log('A');
setTimeout(() => console.log('B'), 0);
Promise.resolve().then(() => console.log('C'));
console.log('D');

What is the correct order of output?

hard

A. A, D, C, B

B. A, B, D, C

C. A, D, B, C

D. A, C, D, B

Solution

1. Step 1: Identify synchronous, microtask, and timers

   console.log('A') and console.log('D') run immediately. Promise.then callbacks run in microtasks after current code. setTimeout callbacks run in timers phase later.

2. Step 2: Determine execution order

   Output order is synchronous logs first (A, D), then microtasks (C), then timers (B).

3. Final Answer:

   A, D, C, B -> Option A

4. Quick Check:

   Microtasks run before timers [OK]

Hint: Promise.then runs before setTimeout even with 0 delay [OK]

Common Mistakes:

• Assuming setTimeout runs before Promise.then
• Mixing synchronous and asynchronous order
• Ignoring microtask queue priority