Travis Horn
Travis Horn

Travis Horn

Delaying forEach() Iterations

Delaying forEach() Iterations

Travis Horn's photo
Travis Horn

Published on Jul 4, 2019

4 min read

I recently ran into the problem of having to loop over an array, but with a delay between iterations. This functionality isn’t built in to JavaScript and I couldn’t find any libraries that provided it out-of-the-box. So I decided to code my own solution. Keep reading and we’ll build it together.

I want to loop through an array of strings and display one string at a time on the page.

We’ll start with a basic HTML template.

<p id="output"></p>

First, let’s create a function which displays text in this &lt;p&gt;.

const output = document.querySelector("#output");
const display = s => output.innerText = s;

Text can be displayed on the page by calling display("Hello World");.

Next, let’s define an array of strings.

const names = [
  "Alberta",
  "Barry",
  "Charley",
  "Christopher",
  "Dianne",
  "Ellen",
  "Ethel",
  "James",
  "Jodee",
  "Joseph",
  "Lilia",
  "Mark",
  "Mary",
  "Merri",
  "Michael",
  "Mildred",
  "Randall",
  "Roy",
  "Thomas",
  "Venus"
];

To loop over this array and display each name, we can use Array.prototype.forEach().

The **forEach()** method executes a provided function once for each array element.

We can loop over the array and call display() on each name.

names.forEach(display);

The problem is that the browser loops over the array so quickly, that only our last element is shown.

We could try slowing it down by adding a simple [setTimeout()](developer.mozilla.org/en-US/docs/Web/API/Wi..).

The **setTimeout()** method sets a timer which executes a function or specified piece of code once the timer expires.

names.forEach((name) => {
  setTimeout(() => {
    display(name);
  }, 1000);
});

But all that happens here is that nothing is displayed for 1000 milliseconds (1 second), then we see the last name again!

But why?

forEach() is looping over our array as fast as possible and setting a 1 second timeout on each item. So they are all going to be executed at the same time: 1 second from now.

What we really need is to set a 0 second timeout on the first item, a 1 second timeout on the second item, a 2 second timeout on the third item, and so on.

To do this, re-write the loop like so.

names.forEach((name, i) => {
  setTimeout(() => {
    display(name);
  }, i * 1000);
});

You can see that instead of setting a static 1000 millisecond delay for all iterations like before, we now multiply the iteration index by 1000 milliseconds.

And it works!

We’ve reached our goal so we could stop here. But we’re really smart so let’s make this more re-usable and functional by moving the function into a delayed-loop factory function.

Start with a blank function that accepts two arguments:

  • another function

  • a delay length

This function will return a function itself.

const delayLoop = (fn, delay) => {
  return 'some function';
};

Copy the existing function we just created earlier into this function as the return value.

const delayLoop = (fn, delay) => {
  return (name, i) => {
    setTimeout(() => {
      display(name);
    }, i * 1000);
  }
};

Instead of the hard-coded display() function call, let’s call whatever function was passed in.

const delayLoop = (fn, delay) => {
  return (name, i) => {
    setTimeout(() => {
      **fn(name);**
    }, i * 1000);
  }
};

Also instead of naming our passed-in argument name we can name it something more modular, like x.

const delayLoop = (fn, delay) => {
  return (**x**, i) => {
    setTimeout(() => {
      fn(**x**);
    }, i * 1000);
  }
};

Finally, instead of hard-coding the value 1000 for the delay length, we can use whatever delay was passed in.

const delayLoop = (fn, delay) => {
  return (x, i) => {
    setTimeout(() => {
      fn(x);
    }, i * **delay**);
  }
};

Our delayed loop function is done. We can pass it to any forEach loop like so. Remember to give it a reference to the display function, as well as an appropriate delay length like 1000.

names.forEach(delayLoop(display, 1000));

All of the JavaScript together, looks like this:

const output = document.querySelector("#output");
const display = s => output.innerText = s;

const names = [
  "Alberta",
  "Barry",
  "Charley",
  "Christopher",
  "Dianne",
  "Ellen",
  "Ethel",
  "James",
  "Jodee",
  "Joseph",
  "Lilia",
  "Mark",
  "Mary",
  "Merri",
  "Michael",
  "Mildred",
  "Randall",
  "Roy",
  "Thomas",
  "Venus"
];

const delayLoop = (fn, delay) => {
  return (name, i) => {
    setTimeout(() => {
      display(name);
    }, i * 1000);
  }
};

names.forEach(delayLoop(display, 1000));

The end result is the same as before.

Now you can use delayLoop() on any forEach() loop. Simply pass the function to use and the amount of time — in milliseconds — to delay between each iteration.

You can see the all the code and a demo on CodePen below.

 
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