I/O callbacks
- Node.js course
- Expert
- Lesson 16/47
- 5 min
Learn I/O callbacks and the poll phase.
I/O callbacks
I/O callbacks run when file and network operations finish. In the poll phase the event loop collects completed I/O events and runs their callbacks. When fs.readFile finishes its callback runs here.
An ioCallbacks(events) function models this phase: the input [{name: 'read'}, {name: 'write'}] returns 'read,write'. events.map(e => e.name) takes each event's name, .join(',') combines them. This shows the poll phase processes I/O events in order. The output is read,write. In real Node.js these callbacks are queued by libuv; the event loop drains them all in the poll phase. The model is deterministic; no timing uncertainty.
Extracting event names
In this lesson we write the ioCallbacks function. The function takes an event array, extracts each event's name property, and joins them with a comma. This models the poll phase processing I/O events in order.
Example: events.map(e => e.name) gives ['read', 'write']. .join(',') yields 'read,write'. console.log(ioCallbacks([{name: 'read'}, {name: 'write'}])) prints read,write. map transforms each item; here from object to name string. join combines the array into a string. These two methods form a common pattern for summarizing the poll phase output. The output is deterministic.
Practice Write the I/O callback function
Write function ioCallbacks(events). Return events.map(e => e.name).join(','). Print console.log(ioCallbacks([{name: 'read'}, {name: 'write'}])).
Starter code
// Write an ioCallbacks function
// Take each event's name
// Join with a comma and print
// Call with [{name: 'read'}, {name: 'write'}]
// write hereSample solution
function ioCallbacks(events) {
return events.map(e => e.name).join(',');
}
console.log(ioCallbacks([{name: 'read'}, {name: 'write'}]));Expected output
read,write
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In which phase does the fs.readFile callback run?
- poll Correct answer
- timers
- check
- close
Explanation: I/O callbacks run in the poll phase.
2 Match each concept to its description
| Term | Meaning |
|---|---|
poll phase |
Processes I/O events |
map |
Transforms each item |
join |
Combines array into string |
fs.readFile |
File read I/O |
3 Complete the map call
return events.____(e => e.name).join(',');return events.map(e => e.name).join(',');Explanation: map turns each event into its name.