JavaScript How to switch between two Intervals in Javascript

Two issues here:

1- In your function change, when you set new intervals, you are not clearing the previous ones, causing the old and new intervals to overlap.

You should clear the current interval before setting the new ones (perhaps you could have one interval variable then?).

2- Your if and else if conditions are the same (count % 2 == 0), so the else block will never execute!

var interval = setInterval(showTime12, 0); //a default format

function change(){
    count++;
    clearInterval(interval); //always clear the prev. interval

    if(count % 2 == 0){
        interval = setInterval(showTime12, 0);
    }
    else {
        setInterval(showTime24, 0);
    }
    console.log(count);
}

Edit: A better approach

As many users pointed out, using a single interval and only changing one little variable, the solution would much be much cleaner. So let's have:

var mode = '12'; //say default format is 12

function change(){
    mode = (mode == '12') ? '24' : '12';
}

function run() {
    action = (mode == '12') ? showTime12() : showTime24();
}

var interval = setInterval(run, 0); //a default format

Answer:1

There is a lot of extra work in the provided code. It's possible to fix the dead branch

if(count % 2 == 0){
    clearInterval(a);
}
else if(count % 2 == 0){ // unreachable!
    clearInterval(b);
}

and use this logic to conditionally set an interval (currently, let a = ... is locally scoped, so after change ends, the previous interval is lost without having been cleared), but an improved design would circumvent the problem entirely.

Instead of using two intervals, a preferred approach would be to continually run one interval and have it perform different kinds of work conditionally.

Additionally, setInterval(fn, 1000) can result in some seconds being skipped since the callbacks may be run more than a second apart. Prefer requestAnimationFrame, which offers better control for constantly-updating functions.

There is a lot of repeated logic in the code. In fact, showTime12 and showTime24 are nearly identical. The helper functions can mostly be inlined without compromising readability.

Here's a working example:

const clock = document.querySelector(".clock");
const changeFormatButton = document.querySelector(".change-format");

let militaryTime = false;
changeFormatButton.addEventListener("click", e => {
  militaryTime = !militaryTime;
});

(function showTime() {
  requestAnimationFrame(showTime);
  const date = new Date();
  let amPm = "";
  let hours = date.getHours();
  
  if (!militaryTime) {
    amPm = hours < 12 ? " AM" : " PM";
    hours = hours % 12 || 12;
  }
  
  clock.innerText = `${hours} : ${date.getMinutes()} : ${date.getSeconds()}`
                    .replace(/\d+/g, m => m.padStart(2, "0")) + amPm;
})();

<div class="clock"></div>
<button class="change-format">change format</button>

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