Event Loop in Action
index.js
To understand how the Node.js event loop schedules tasks, examine the code sample above. The script begins with two synchronous console.log statements, then schedules a setTimeout, an asynchronous file read, and a process.nextTick callback. The execution order is determined by the event loop's rules for handling synchronous code, microtasks like process.nextTick, timers, and I/O callbacks.
First, Node.js runs all top-level synchronous code. This means both console.log('A: Synchronous start') and console.log('E: Synchronous end') execute immediately. While these lines run, the setTimeout, fs.readFile, and process.nextTick are scheduled for later execution.
Next, process.nextTick callbacks are always executed right after the current operation completes, before the event loop continues to other phases. Thus, "D: process.nextTick" is printed after the synchronous code but before any timers or I/O callbacks.
The setTimeout callback is placed in the timer phase and will run as soon as possible after a minimum delay (here, zero), but only after microtasks like process.nextTick finish. The file read with fs.readFile is asynchronous and its callback is queued in the poll phase, which may run after timers depending on system conditions and timing.
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Can you explain the exact output order of the console logs in this script?
What is the difference between process.nextTick and setTimeout in the event loop?
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index.js
To understand how the Node.js event loop schedules tasks, examine the code sample above. The script begins with two synchronous console.log statements, then schedules a setTimeout, an asynchronous file read, and a process.nextTick callback. The execution order is determined by the event loop's rules for handling synchronous code, microtasks like process.nextTick, timers, and I/O callbacks.
First, Node.js runs all top-level synchronous code. This means both console.log('A: Synchronous start') and console.log('E: Synchronous end') execute immediately. While these lines run, the setTimeout, fs.readFile, and process.nextTick are scheduled for later execution.
Next, process.nextTick callbacks are always executed right after the current operation completes, before the event loop continues to other phases. Thus, "D: process.nextTick" is printed after the synchronous code but before any timers or I/O callbacks.
The setTimeout callback is placed in the timer phase and will run as soon as possible after a minimum delay (here, zero), but only after microtasks like process.nextTick finish. The file read with fs.readFile is asynchronous and its callback is queued in the poll phase, which may run after timers depending on system conditions and timing.
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