function a() {
    setTimeout(b, 3000);
    console.info('This is a!');
}
function b() {
    setTimeout(a, 3000);
    console.info('This is b!');
}
a();

function start() {
    a()
}
var timer;
function a() {
    clearTimeout(timer);
    timer = setTimeout(b, 3000);
}
function b() {
    clearTimeout(timer);
    timer = setTimeout(a, 3000);
}

a and b are executed every six seconds, a is executed first, and b is executed every three seconds. . . Otherwise, if the function execution time is taken into account, the three seconds interval is not the exact three seconds

var flag = true;
var s = setInterval(function(){
    if(flag){
        a();
        
    }else{
        b()
    }
    flag =!flag

},3000)