timing for loop in JS

Question

I wrote this code in JS:

function startFunction() {
p1 = document.getElementById('p1').innerHTML;
for (var i=1; i<=p1.length; i++) {
        alert(p1.slice(0, i));
    }
}

I call the function with onload event in html:

    <body onload="startFunction()">

And thi is the paragraph with p1 id:

    <p id="p1">Hi, I'm</p>

How can I make a delay for the for loop. I want my program to write the p1 text letter by letter.

Possible duplicate of [Letter by letter animation with delay in loading](https://stackoverflow.com/questions/39701852/letter-by-letter-animation-with-delay-in-loading) — devlin carnate, Nov 13 '18 at 18:29
@devlincarnate dupes may exist, but dig up one without jQuery please. — tevemadar, Nov 13 '18 at 18:30
@tevemadar - the gist of the answer isn't using jQuery. In fact, it's nearly identical to the first comment above. — devlin carnate, Nov 13 '18 at 18:32

score 1 · Answer 1 · answered Nov 13 '18 at 18:37

var alertEachLetter =function(p1, i){
    setTimeout(function(){
        alert(p1.slice(0, i));
    },1000);
};

 function startFunction() {
 p1 = document.getElementById('p1').innerHTML;
 for (var i=1; i<=p1.length; i++) {
        alertEachLetter(p1, i);
    }
}

why create this alertEachLetter function. for that you need to check this link

setTimeout in for-loop does not print consecutive values

score 1 · Accepted Answer · answered Nov 13 '18 at 18:37

You can not and should not delay anything inside a loop, because that is how the nonresponsive pages are made: the browser does not react to user actions or do anything visible until the JavaScript code returns.

Instead, you can use some timer, like setInterval():

function startFunction() {
    var p1 = document.getElementById('p1');
    var txt = p1.innerHTML;
    var i=0;
    var timer = setInterval(function() {
        p1.innerHTML = txt.slice(0,i++);
        if(i>txt.length) {
            clearInterval(timer);
        }
    },500);
}

startFunction();

<p id="p1">Hi, I'm</p>

score 1 · Answer 3 · answered Nov 13 '18 at 18:39

You don't need a loop, you need an interval. Javascript's interval feature will call your function at (approximately) the requested interval. So, for example:

function startFunction() {
  var p1 = document.getElementById('p1').innerHTML
  var count = 1
  var finished = p1.length
  var iv = setInterval(function() {
    alert(p1.slice(0,count++))
    if (count > finished) {
      clearInterval(iv) // stops the interval from firing once we finish our task
    }
  }, 1000) // 1000 ms, or every second.
}

Andy · Answer 4 · 2018-11-13T21:24:45.853

Here's a quick example using setTimeout instead of setInterval. There's not much difference except you don't have to clear the timeout - you simply don't run it if it doesn't meet a condition.

// cache the elements
const p1 = document.getElementById('p1');
const out = document.getElementById('out');

// make the text content from p1 iterable and split it into
// the head (first element), and tail (everything else)
const [head, ...tail] = [...p1.textContent];

const loop = function loop(head, tail) {

  // update the output text content with the result of head
  out.textContent = head;

  // if there's anything left of the tail array
  if (tail.length) {

    // remove the first element of tail and
    // add it to head
    head += tail.shift();

    // call the function again with the new head and tail
    setTimeout(loop, 200, head, tail);
  }

// pass in the head and tail to the function
}(head, tail);

#p1 { display: none; }

<p id="p1">Content written letter by letter</p>
<p id="out"></p>

Tom O. · Answer 5 · 2018-11-13T18:58:34.473

Below is an approach I think may help you achieve what youre trying to do. This approach uses setInterval (instead of a loop) to execute a function multiple times. See the comments to understand the code logic:

//Grab our DOM elements
var p1 = document.getElementById('p1').innerHTML;
var copy = document.getElementById('copy');

//Execute a function every 250 milliseconds
var intervalId = setInterval(onInterval, 250);

//nextLetter is a function that will return the character at a particular index in the string. The function will increase the index each time it is called. The function will return null once it exceeds the innerHTML length. c is a "private" variable that can't be modified elsewhere in the program.
var nextLetter = (function(i, limit) {
  var c = i;
  return function() {
    var idx = c++;
    if (idx > limit) {
      return null;
    }
    return p1.charAt(idx);
  };
})(0, p1.length);

//The function we will execute at each interval
function onInterval() {
  var letter = nextLetter();
  if (letter) {
    copy.innerHTML += letter;
  } else {
    console.log('End of content reached - removing interval');
    clearTimeout(intervalId);
  }
}

<p id="p1">Make sure to read the in-code comments</p>
<p id="copy"></p>

timing for loop in JS

5 Answers5