Summary

In this lesson, you learned that:

• setTimeout(function () { ... }, 2000) runs the function once, 2000 milliseconds (2 seconds) later. Pass the function itself, without ().
• The script doesn't wait for a timer: the lines after setTimeout() run straight away, so everything that should happen after the wait goes into the function.
• setTimeout() returns the timer's id, and clearTimeout(timer) cancels the timer. Keep the id in a let variable outside the handlers, so that every handler can use it.
• setInterval(function () { ... }, 1000) runs the function every second until clearInterval(timer) stops it. A variable outside the function remembers what changes between the runs, such as the seconds left.
• button.disabled = true; disables a button while the page is busy, and false enables it again.
• A function that takes a while returns a Promise, a value that arrives later. await waits for the value, and it works only in a function marked async. Without await, you get the Promise itself, and the page shows [object Promise].
• An async function can wait several times, one step after another, for example with await wait(1000);. The rest of the page keeps working in the meantime.
• A message such as Loading… before the await tells the user that the page is waiting.
• When a Promise fails, await throws an error. try { ... } catch (error) { ... } catches it, and error.message is its text. finally runs at the end in both cases.

In the next lesson, Data from a Server, your pages will load data from a server with fetch(), which returns a Promise too, so you'll use await there all the time.

But first, some practice with timers: a lightning quiz, in which the player has just 2 seconds to answer. This time, script.js is empty, and you'll write the whole program yourself.