one div gets bigger while the other gets smaller

Question

I\'m new to javascript, I\'m wondering how to get .contact from a 30% width to 40% when I go with my mouse over .contact. This works, but while .contact gets bigger I want .

Answer 1

You can do it with Flexbox

flex: 1 makes .contact expand to the rest of the available space.

You would then only need to define the width for .div and its width when .contact is hovered.

.content {
  display: flex;
}

.contact{
  background-color:red;
  flex: 1
}

.div {
  width: 70%;
  background-color: blue;  
}

.contact:hover + .div {
  width: 60%
}

<div class="content">       
  <div class="contact" id="contact">                    
    <h3> Text</h3>
    <p>More textt</p>
  </div>
  <div class="div">
    <h3> Text</h3>
    <p>More textt</p>
  </div>                
</div>

Answer 2

You can do this with only CSS

.contact {
  background-color: red;
  width: 30%;
  float: left;
  transition:1s linear;
}

.div {
  background-color: blue;
  width: 70%;
  float: right;
  transition:1s linear;
}

.contact:hover {
  width: 40%;
}

.contact:hover + .div{
  width: 60%;
}

<div class="content">

  <div class="contact" id="contact">
    <h3> Text</h3>
    <p>More textt</p>
  </div>

  <div class="div">
    <h3> Text</h3>
    <p>More textt</p>
  </div>

</div>

And for a more flexible way you can consider flexbox where you only need to change the hover element and the other one will shrink by default

.content {
  display: flex;
}

.contact {
  background-color: red;
  flex-basis: 30%;
  transition: 0.5s linear;
}

.div {
  background-color: blue;
  flex-grow:1;
}

.contact:hover {
  flex-basis: 40%;
}

<div class="content">

  <div class="contact" id="contact">
    <h3> Text</h3>
    <p>More textt</p>
  </div>

  <div class="div">
    <h3> Text</h3>
    <p>More textt</p>
  </div>

</div>

UPDATE

If you want a permanent change you can try animation:

.content {
  display: flex;
}

.contact {
  background-color: red;
  flex-basis: 30%;
  transition: 0.5s linear;
  animation:big 0.5s linear forwards;
  animation-play-state:paused;
}

.div {
  background-color: blue;
  flex-grow:1;
}

.contact:hover {
  animation-play-state:running;
}
@keyframes big{
  to {flex-basis: 40%;}
}

<div class="content">

  <div class="contact" id="contact">
    <h3> Text</h3>
    <p>More textt</p>
  </div>

  <div class="div">
    <h3> Text</h3>
    <p>More textt</p>
  </div>

</div>

Answer 3

You don't need javascript for this, sibling selector works.
Or in javascript, shrink the div on the right while expanding the div on the left.

var width = 30;

var maxWidth = 40;

var interval = null;

var contact = document.getElementById("contact");
var div = document.getElementById("div");
function myMove() {
  interval = setInterval(function() {
    if (width>= maxWidth){
      return clearInterval(interval);
    }
    contact.style.width = ++width + "%";
    div.style.width = (100-width) + "%";
  },5);
}

.contact{
    background-color:red;
    width:30%;
    float:left;
}
.div{
    background-color: blue;
    width: 70%;
    float: right;  
}

<div class="content">
                
        <div class="contact" id="contact" onmouseover="myMove()">                    
            <h3> Text</h3>
            <p>More textt</p>
        </div>

        <div class="div" id="div">
            <h3> Text</h3>
            <p>More textt</p>
        </div>                

</div>