// Timeout is cleared immediately.
var t = setTimeout(function() {
  console.log("The timeout has completed");
}, 10000);
console.log("Timer ID: " + t);
clearTimeout(t);

Example: Stop the Countdown The user controls whether to clear the 3 second timeout timer.

// The user controls whether to clear the 3 second timeout timer.
textLabel("instructions", "Click Start to begin a 3 second timeout timer, then Stop to prevent it from completing");
textLabel("status", "");
button("startButton", "Start");
button("cancelButton", "Stop");
var t;
onEvent("startButton", "click", function(){
  t = setTimeout(function() {
    setText("status", "The timer completed!");
   }, 3000);
   setText("status", "Timer started!");
   console.log("Timer ID: " + t);
});
onEvent("cancelButton", "click", function(){
  if(t) {
    clearTimeout(t);
    setText("status", "The timer has been cancelled.");
  } else {
    setText("status", "You need to start the timer before you can stop it :)");
  }
});