Event loop phases and timer execution in Node.js - Step-by-Step Execution

Concept Flow - Event loop phases and timer execution

Start Event Loop

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Timers Phase

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Pending Callbacks Phase

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Idle, Prepare Phase

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Poll Phase

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Check Phase

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Close Callbacks Phase

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Repeat Event Loop

The Node.js event loop runs in phases, each handling specific callbacks like timers, I/O, and close events, cycling continuously.

Execution Sample

Node.js

setTimeout(() => console.log('timer1'), 0);
setImmediate(() => console.log('immediate1'));
console.log('start');

This code schedules a timer and an immediate callback, then logs 'start' immediately.

Execution Table

Step	Phase	Action	Callback Executed	Output
1	Main script	Execute synchronous code	None	start
2	Timers phase	Execute timer callbacks	timer1 callback	timer1
3	Pending Callbacks	No callbacks	None
4	Idle, Prepare	Internal setup	None
5	Poll phase	Check for I/O callbacks	None
6	Check phase	Execute immediate callbacks	immediate1 callback	immediate1
7	Close callbacks	No callbacks	None
8	Event loop repeats	Cycle continues	Depends on new events

💡 Event loop runs continuously until no more callbacks or process exit

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 6	Final
Timers queue	empty	timer1 scheduled	timer1 executed	empty	empty
Immediates queue	empty	immediate1 scheduled	immediate1 pending	immediate1 executed	empty
Console output		start	start, timer1	start, timer1, immediate1	start, timer1, immediate1

Key Moments - 3 Insights

Why does 'start' print before 'timer1' even though timer1 has 0 delay?

'start' is logged during the main script execution (Step 1) before the event loop phases begin. Timer callbacks run only in the Timers phase (Step 2), so they come after synchronous code.

Why does 'immediate1' print after 'timer1' even though setImmediate is called after setTimeout?

setTimeout callbacks run in the Timers phase (Step 2), while setImmediate callbacks run in the Check phase (Step 6). The event loop processes Timers phase before Check phase, so timer1 prints first.

Can the order of timer and immediate callbacks change?

Yes, depending on when they are scheduled and the current phase of the event loop. But in this example, timers run before immediates consistently.

Visual Quiz - 3 Questions

Look at the execution table, what is printed at Step 1?

A "immediate1"

B "timer1"

C "start"

D Nothing

💡 Hint

Check the 'Output' column for Step 1 in the execution_table

At which step does the 'immediate1' callback execute?

A Step 6

B Step 1

C Step 2

D Step 4

💡 Hint

Look for 'immediate1 callback' in the 'Callback Executed' column

If setTimeout delay was increased, how would that affect the execution order?

A timer1 would run before immediate1

B immediate1 would run before timer1

C Both run at the same time

D Neither would run

💡 Hint

Consider the event loop phases and when timers vs immediates run

Concept Snapshot

Node.js event loop cycles through phases:
Timers (setTimeout/setInterval callbacks)
Pending Callbacks (I/O callbacks)
Idle, Prepare (internal)
Poll (I/O polling)
Check (setImmediate callbacks)
Close Callbacks
Timers run before immediates
Synchronous code runs before event loop phases

Full Transcript

The Node.js event loop runs continuously in phases. First, synchronous code runs immediately. Then the Timers phase executes callbacks scheduled by setTimeout or setInterval. Next phases handle I/O callbacks and internal tasks. The Check phase runs callbacks scheduled by setImmediate. This cycle repeats until no callbacks remain. In the example, 'start' logs first during synchronous execution. Then the timer callback runs in the Timers phase, printing 'timer1'. Finally, the immediate callback runs in the Check phase, printing 'immediate1'. This order is consistent because timers run before immediates in the event loop phases.