Francesco Esposito

Posted on Feb 2, 2019 • Updated on Apr 21, 2023

Two-way data binding in vanilla JS (POC)

#showdev #javascript #webdev #databinding

As Front-end Engineers we mostly use libraries and/or frameworks to develop and maintain complex web apps, but what there is under the hood? Do you ask yourself that question? You don't!? Well, You should! 🙃

In this post, I would like to extend the previous example to two-way data binding. 🕺

Two-way data binding 🤓
If you are not familiar with that concept, two-way data binding means that every change to the state is immediately propagated to the view (and vice-versa).

Check out the demo below for a quick example:

DEMO

Let's break it down

What do we need to have two-way data binding?

A view, in our example HTML.
A state, kept in memory with JavaScript.

The Key feature is:

Every time the state changes, the view needs to be updated (one-way data binding)

but also

Every time the view changes, the state needs to be updated

So let's assume we have an HTML view:

    <div class="field">
        <label for="name">Enter your name:</label>
        <input id="name" type="text" name="name"  data-model="name" />
      </div>

      <div class="field">
        <label for="title">Enter your title:</label>
        <input id="title" type="text" name="title" data-model="title" />
      </div>

      <div class="results">
        <h1 data-binding="name"></h1>
        <h2 data-binding="title"></h2>
      </div>

and a state:

    const state = {
      name: 'Francesco',
      title: 'Front-end Developer'
    };

We can easily set the view the first time:

    document.querySelector('[data-binding="name"]').innerHTML = state.name
    document.querySelector('[data-binding="title"]').innerHTML = state.title
    document.querySelector('[data-model="name"]').value = state.name
    document.querySelector('[data-model="title"]').value = state.title

But we want some magic, so that when we update the state:

    state.name = 'Richard'
    state.title = 'Technical Lead'

the view should update too.

To achieve this, we could modify the default behaviour of the set property for the state object, so that other than update the state, it would also update our view.

One way to do that in JavaScript is using the Proxy Object:

    const createState = (state) => {
      return new Proxy(state, {
        set(target, property, value) {
          target[property] = value; // default set behaviour
          render(); // updates the view every time the state changes
          return true;
    }
      });
    };

    const state = createState({
      name = 'Francesco'
      title = 'Front-end Engineer'
    });

With the power of the Proxy every time we update our state, the render function will be called.
A possible implementation of render can be:

    const render = () => {
     document.querySelector('[data-binding="name"]').innerHTML = state.name;
     document.querySelector('[data-binding="title"]').innerHTML = state.title;
     document.querySelector('[data-model="name"]').value = state.name;
     document.querySelector('[data-model="title"]').value = state.title;
    };

We just miss the last little piece. Every time we modify the view, the state should change accordingly. We can obtain that adding an event listener to the inputs: 😎

    const listener = (event) => {
      state[event.target.dataset.model] = event.target.value;
    });

   document.querySelector('[data-model="name"]').addEventListener('keyup', listener);  
   document.querySelector('[data-model="title"]').addEventListener('keyup', listener);

And Voilá! Now the trick is complete! 👨‍💻

More generic implementation (POC) 🌟

Top comments (8)

Tormod Vold Mikkelsen • Nov 15 '19

Very helpful! Thanks a lot. :)
You have a typo in there:

    const state = createState({
      name = 'Francesco'
      title = 'Front-end Engineer'
    });

Should be

    const state = createState({
      name: 'Francesco',
      title: 'Front-end Engineer'
    });

Anuj jaryal • Oct 9 '21

How extend it to include array of elements, like
I have in DOM:

< input name=product[1][quantity]>
and so on...

Francesco Esposito • Nov 10 '21

I would say that this is mostly a Proof of concept to show a possible implementation of two-way data binding, but for an app I would advice you to use a library (e.g. React)

Brian • Apr 4 '20

Thanks for the article. How would you expand on this?
Unless I am understanding it wrong, It looks like the render function updates everything that has a binding. Would you make it so that it only updates the bindings that had changes?

Sushil Kundu • Jun 8 '20 • Edited

Yes. You are right. Here is the updated code.

const state = createState({
name: '',
title: ''
});

const render = (property) => {
document.querySelector([data-model=${property}]).value = state[property];
};

const listener = (event) => {
const {type, value, dataset} = event.target;
state[dataset.model] = value;
};

document.querySelector('[data-model="name"]').addEventListener('keyup', listener);

document.querySelector('[data-model="title"]').addEventListener('keyup', listener);