JAVASCRIPT TIMER CLOCK CODE

Creating this will help you know about accessing the DOM, adding activity to them and much more.

Disables the stop button when there is nothing to stop.Building a digital clock using JavaScript can be an excellent project for a beginner to understand the basic concepts of JavaScript.

(note the last event will make changes to the DOM)

Checks the page visibility before making visual changes to the page.

Listens to visibilit圜hange so that the time the client sees is always correct.

Adds event listeners rather than set event listeners.

(note that I was lazy and only set the seconds) Also sets the machine readable property dateTime.

Uses the correct element to display time.

Calculates time till next visual change rather than use unreliable interval timer.

Calculates timer value rather than count time using timer events.

JAVASCRIPT TIMER CLOCK CODE

Never set event listeners directly eg onclick='somefunction' as the event listener can be overwritten by accident by you, or by poorly written 3rd party code or extensionsĪlways use EventTarget.addEventListener to add event listeners.Ī button that does nothing should be disabled.įollowing points outlined above the snippet implements a countdown timer with start/restart and stop buttons. To measure time till an event store the expected time and calculate the time from now till then.Īvoid using setInterval as it can fire well off the expected interval, rather calculate the time until the next visual change and use setTimeoutĪvoid using let in the global scope as its scope is un-intuitive and may throw errors due to clashes with existing named variables unrelated to you code. Timers such as setInterval and setTimeout should not be use to measure time. Use Node.textContent and avoid the expensive re-flow. Never use Element.innerHTML to set text only content as it will force a CPU/GPU expensive re-flow.

This gives meaning to the content and lets machines understand what the content represents. To display a time such as a count down use the HTMLTimeElement and set its HTMLTimeElement.dateTime property to match the visible countdown time. Use the page visibility API to check for page visibility and listen to visibility change events. Timers are one such app that contains animations that the client will not continually watch. In timers you should check if the document is visible before you make changes that may not be viewed by the client. Some points missed or skimmed over by other answers.Īlways keep DOM interaction to a minimum. Why console.clear()? Production code should avoid console.since clearInterval returns undefined and because it makes sense, I would go for startTimer = clearInterval(startTimer).I am not sure why you return anything in startStopTimer? If you are never using the returned value, then just drop the return keyword.

Your entire code should be in a well named (self executing) function.

Placing the timer in a h1 element is probably not a good idea semantically.

You are supposed to keep track of the original start and deduct that from current time to determine how much time has passed.

The browser tries to honor the 1000, but it cannot be trusted.

Similarly, 00:0$ does not allow for much flexibility in the script.

I would expect to be able to change 9 to 99 and to have the timer start at 01:40 You hard code both 9 and 00:10, this is not flexible.You use 9 twice, this should be assigned to a nicely named global constant.

Javascript timer clock