How to make this illumination effect with CSS

[Leader Leader]

simulate a “scan” light that will reveal words in a box, this is my code by now:

const e = document.getElementsByClassName('scan')[0];
document.onmousemove = function(event){
  e.style.left = `${event.clientX}px`;
};

*{
    margin: 0;
    padding: 0;
}

html, body{
    width: 100%;
    min-height: 100vh;
    overflow-x: hidden;
    
    display: flex;
}

.banner{
    width: 100vw;
    height: 100vh;

    display: flex;
    flex-grow: 1;
    flex-direction: row;
    align-items: center;
    background-color: #031321;
}

.banner .scan{
    width: 7px;
    height: 80%;
    
    position: absolute;
    left: 30px;
    z-index: 3;

    transition: left 50ms ease-out 0s;
    
    border-radius: 15px;
    background-color: #fff;
    box-shadow:
        0 0 15px 5px #fff,  /* inner white */
        0 0 35px 15px #008cff, /* inner blue */
        0 0 350px 20px #0ff; /* outer cyan */
}

.banner .description{
    width: 100%;
    color: white;
    font-size: 3em;
    text-align: center;

    -webkit-user-select: none;
    -ms-user-select: none;
    user-select: none;
}

<div class="banner">
    <div class="scan"></div>
    <div class="description">
        Just trying something
    </div>
</div>

1.mp4

[zaasmi]

You can increase the background-size (500px in this example) to increase transition smoothing.

const e = document.getElementsByClassName('scan')[0];
const hidden = document.getElementsByClassName('hidden')[0];

document.onmousemove = function(event) {
  e.style.left = `${event.clientX}px`; //               ↓ background width (500px) / 2
  hidden.style.setProperty("--pos", `${event.clientX - 250}px`);
};

* {
  margin: 0;
  padding: 0;
}

html,
body {
  width: 100%;
  min-height: 100vh;
  overflow-x: hidden;
  display: flex;
}

.banner {
  width: 100vw;
  height: 100vh;
  display: flex;
  flex-grow: 1;
  flex-direction: row;
  align-items: center;
  background-color: #031321;
}

.banner .scan {
  width: 7px;
  height: 80%;
  position: absolute;
  left: 30px;
  z-index: 3;
  transition: left 50ms ease-out 0s;
  border-radius: 15px;
  background-color: #fff;
  box-shadow: 0 0 15px 5px #fff, /* inner white */
  0 0 35px 15px #008cff, /* inner blue */
  0 0 350px 20px #0ff;
  /* outer cyan */
}

.banner .description {
  width: 100%;
  color: white;
  font-size: 3em;
  text-align: center;
  -webkit-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

.hidden {
  background: radial-gradient(dodgerblue 10%, #031321 50%) var(--pos) 50% / 500px 500px no-repeat fixed;
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
}

<div class="banner">
  <div class="scan"></div>
  <div class="description">
    Just <span class="hidden">hidden</span> something
  </div>
</div>