setTimeout vs setInterval in JavaScript

setTimeout(fn, delay) schedules fn to run once after delay milliseconds. setInterval(fn, delay) schedules fn to run repeatedly every delay milliseconds until explicitly stopped.

Common causes

• setInterval doesn't account for how long fn itself takes to run — if fn takes longer than delay, calls can queue up or overlap in some environments
• Both timers are minimums, not guarantees — the browser or Node event loop can delay execution if the main thread is busy

How to fix it

• For repeating work where drift matters, prefer a recursive setTimeout — schedule the next call only after the current one finishes
• Always store the return value (a timer ID) and call clearTimeout()/clearInterval() when the timer is no longer needed, especially in component unmount/cleanup code
• Use setInterval only for simple, lightweight repeating tasks where slight timing drift is acceptable

Example

function poll() {
  doWork()
  setTimeout(poll, 1000)
}
poll()

FAQ

Why is recursive setTimeout better than setInterval for polling?

It guarantees the delay is measured from when the previous call finished, not from a fixed schedule — so slow work never causes overlapping executions.