How to Change Interval Time Dynamically in For Loop According to Index/iteration Number?

While this task could be solved with promises, reactive streams and other cool tools (hey, nobody has suggested using workers yet!), it can also be solved with a little arithmetics.

So you want timeouts in a sequence: 1s, the previous one + 2s, the previous one + 3s, and so on. This sequence is: 1, 3, 6, 10, 15... and its formula is a[n] = n * (n + 1) / 2. Knowing that...

let n = 6;
console.log(new Date().getSeconds());

for (let i = 1; i < n; i++) {
  setTimeout(function timer() {
    console.log(new Date().getSeconds());
  }, 1000 * i * (i + 1) / 2);
}

Here is a function that will show immediately, then 1 second later, 2 seconds after than, 3 seconds after that etc. No special math, no promises needed

const n = 5;
let cnt=0;

function show() {
  console.log("call "+cnt,"delay: ",cnt,"sec");
  cnt++;
  if (cnt > n) return; // we are done
  setTimeout(show, cnt*1000 ); // cnt seconds later
}
show()

Took me some time to decipher your question xD, but is this what you want?

This will keep firing console.log with i*1000 delay each time. so the first time it will be 1 second long (1*1000), next it will be 2 seconds and so on.

let i = 0;
loop = () => {
  setTimeout(() => {
    console.log(new Date()); // for clarity
    i++;
    if (i < 10) {
      loop();
    }
  }, i * 1000)
};
loop();