Loop over array with setInterval doesn't work

For a small amout of intervals, I suggest to use setTimeout instead and given them appropriate timeouts and a closure over the wanted character to add to innerHTML of the element.

function loop(word, target) {
    for (let i = 0; i < word.length; i++) {
        setTimeout((c => () => target.innerHTML += c)(word[i]), (i + 1)* 1000);
    }
}

loop('hello', document.querySelector('h1'));

<h1></h1>

If you still want to use setInterval, you need to store the interval and clear this interval at the end of the word.

function loop(word, target) {
    var interval = setInterval((i => () => {
        if (i >= word.length) return clearInterval(interval);
        target.innerHTML += word[i++];
    })(0), 1000);
}

loop('hello', document.querySelector('h1'));

<h1></h1>

This is because you're doing the interval inside the loop, that means by the time you get to your 1s the i is already at the end of your array i = 4.

I think this is what you want?

let i = 0;
let interval;

const loop = () => {
  console.log(word[i]);
  if (word[i]) title.textContent += word[i];
  else clearInterval(interval)
  i ++;
}

interval = window.setInterval(loop, 1000);

Hope this helps

If you want to change your title after 1 second for all the words in array you can start with index 0 and keep on setInterval till your index is less than word.length.

Like:

const h2 = document.querySelector('h2');
const title = document.querySelector('h1');

const word = ['h', 'e', 'l', 'l', 'o'];

loop(0);

function loop(index) {
  setTimeout(() => {
    if (index < word.length) {
      title.textContent = word[index];
      setTimeout(() => loop(index+ 1), 1000);
    } else {
    setTimeout(() => loop(0), 1000);
    }
  }, 1000)
}

Here I am setting the first element, and doing setTimeout for the next one

if index has exceeded the word length I have put else as well to reset the word loop.

Here is my understanding. This program should append hello for every one second.

This code works for me

html

<h1 />
<h2 />

js

const h2 = document.querySelector('h2');
const title = document.querySelector('h1');

const word = ['h', 'e', 'l', 'l', 'o'];

function loop() {
   for(let i = 0; i < word.length; i++) {
       window.setInterval(() => {
            console.log(word[i]);
            title.textContent += word[i] ;
       }, 1000)
    }
}

loop();

demo:

https://jsfiddle.net/Danielprabhakaran_N/sjzrx3qy/