2 way data binding in JavaScript

When an input is changed update the value, add a setter to the value which sets the inputs content. E.g this element:

<input id="age">

And some js:

var person = (function(el){
 return {
   set age(v){
    el.value = v;
   },
   get age(){
     return el.value;
   }
 };
})(document.getElementById("age"));

So you can do:

 person.age = 15;

And the input will change. Changing the input changes person.age

Simple and working approach to two-way binding, only using vanilla JS.

<!-- index.html -->
<form action="#" onsubmit="vm.onSubmit(event, this)">
    <input onchange="vm.username=this.value" type="text" id="Username">
    <input type="submit" id="Submit">
</form>
<script src="vm.js"></script>

// vm.js - vanialla JS
let vm = {
    _username: "",
    get username() {
        return this._username;
    },
    set username(value) {
        this._username = value;
    },
    onSubmit: function (event, element) {
        console.log(this.username);
    }
}

JS Getters and Setters are quite nice for this - especially when you look at the browser support.

Yes, we can achieve the two way data binding using pure javascript.

twoWay=function(event) {
  var elem = document.getElementsByClassName(event.currentTarget.className);
  for(var key in elem){
    elem[key].value=event.currentTarget.value;
  }
}

You can check the jsfiddle.

Yes indeed. There are frameworks like angular Js which provides full support for two way data binding. And if you want to achieve the same in vanilla js you can bind value into view

Eg. document.getElementById('test').value="This is a Test"

And to bind view value to the controller you can trigger onchange event in html.

 <Input type="text" id="test" onchange="Func()">

Adding a little elaboration to Jonas Wilms answer, here's a sample without currying and also adds event binding for a full two way bind.

// Property binding
var person = { 
  set name(v) {
    document.getElementById('name').value = v;
  },
  get name() {
    return document.getElementById('name').value;
  },
  set age(v) {
    document.getElementById('age').value = v;
  },
  get age() {
    return document.getElementById('age').value;
  }
};

// You can now set values as such
person.name = 'Cesar';
person.age = 12;

// Event binding completes the two way
function logToConsole(event) { 
  console.log(event.target.value);
}

// You can set person.name or person.age in console as well.

<label for="name">Name: </label><input id="name" onkeyup="logToConsole(event)">
<label for="age">Age: </label><input id="age" onkeyup="logToConsole(event)">

Would you mind if it would be a small component for databinding tasks that provides enough convenient databinding definition commands. I did it with databindjs. e.g.

// Lets assume that there is just simple form (target)
var simpleForm = {
   input: $('.simple .input-value'),
   output: $('.simple .output-value')
};
// And here is the simple model object (source)
var model = {
    text: 'initial value'
};

// Lets set two directional binding between [input] <-> [text]
var simpleBinding = bindTo(simpleForm, () => model, {
    'input.val': 'text',  // bind to user input
    'output.text': 'text'  // simple region that will react on user input
});
// This command will sync values from source to target (from model to view)
updateLayout(simpleBinding);
subscribeToChange(simpleBinding, () => {
    $('.simple .console').html(JSON.stringify(model));
});
// Just initialize console from default model state
$('.simple .console').html(JSON.stringify(model));

The full solution here. You can check the full implementation of the databinding core on github