Javascript Sandbox:

Inspired by Simone Vittori's real time editor.

Console log:

//Execute by pressing CTRL!
log.textContent = '' // Clear the console log.

var startingHeight = 25
var fallTime = 0
console.log('Your hailstone starts at ' + startingHeight + ' meters... <br>Hailstone algorithm... <br>GOOOOOO!!!')

function hailstone(h) {
    fallTime++
    console.log(h + ' m')
    if (h <= 1) {
        console.log('Thunk!<br>Your hailstone hit the ground after ' + fallTime + ' seconds.')
        return
    }
    else {
        // Divide by 2 if h is even, otherwise multiply by 3 and add 1:
        if (h % 2 == 0)
            h /= 2
        else
            h = h*3 + 1
        // Sleep for 100 milliseconds, then run this function again:
        setTimeout(() => {
          hailstone(h)
        }, 100);
    }
    
}

hailstone(startingHeight)

// Code run after hailstone() will run before the timeouts are finished!
// This is because setTimeout() puts the calls that hailstone() makes to itself on a different thread.
// This is also why terminating the main thread doesn't stop the hailstone.