Node.js
Timers queue
Learn the timers queue and setTimeout ordering.
Timers queue
The timers queue holds callbacks registered with setTimeout and setInterval. In the timers phase the event loop runs the timers whose deadline has passed. Timers are ordered by delay; smaller delays run first.
A timersByDelay(delays) function models this ordering: the input [100, 10, 50] returns '10,50,100'. delays.slice().sort((a, b) => a - b) first copies the array (to avoid mutating the original), then sorts numerically. .join(',') turns it into a string. This shows that the timers phase runs callbacks in delay order. The output is 10,50,100; the smallest delay first.
Numeric sorting
JavaScript sort defaults to string sorting; it orders numbers incorrectly ([10, 100, 50] becomes [10, 100, 50]). For numeric sorting you pass a comparator: (a, b) => a - b. This sorts ascending.
Example: [100, 10, 50].sort((a, b) => a - b) gives [10, 50, 100]. We take a copy with delays.slice() because sort mutates in place. .join(',') yields the string '10,50,100'. This models the timers phase running callbacks in correct delay order. There is no timer uncertainty; the ordering is deterministic. The output is 10,50,100.
Exercise Write the timers ordering
Write function timersByDelay(delays). Return delays.slice().sort((a, b) => a - b).join(','). Print console.log(timersByDelay([100, 10, 50])).
Show example solution
function timersByDelay(delays) {
return delays.slice().sort((a, b) => a - b).join(',');
}
console.log(timersByDelay([100, 10, 50]));
Test yourself
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Complete the sort call
return delays.slice().____((a, b) => a - b).join(',');
Show answer
return delays.slice().sort((a, b) => a - b).join(',');
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What does [100, 10, 50].sort((a, b) => a - b) give?
[100, 10, 50].sort((a, b) => a - b)
- [100, 50, 10]
- [10, 100, 50]
- [10, 50, 100]
- [50, 10, 100]
Show answer
[10, 50, 100] — a - b sorts ascending: [10, 50, 100].
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Match each concept to its description
- setTimeout
- One-shot timer
- sort
- Orders an array
- slice
- Takes a copy of an array
- timers phase
- Runs timers
Next lesson: I/O callbacks
