That happens to be how common browsers have implemented it but the spec does not guarantee it: > Let handle be a user-agent-defined integer that is greater tha

That happens to be how common browsers have implemented it but the spec does not guarantee it: > Let handle be a user-agent-defined integer that is greater than zero that will identify the timeout to be set by this call in the list of active timers. https://dev.w3.org/html5/spec-preview/timers.html https://dev.w3.org/html5/spec-preview/timers.html Edit: Just noticed your edit.