Real Viewport (No more issues with the 100vh in mobile browsers)

real-viewport.js

/* Vh Calc */
// First we get the viewport height and we multiple it by 1% to get a value for a vh unit
let vh = window.innerHeight * 0.01;
// Then we set the value in the --vh custom property to the root of the document
document.documentElement.style.setProperty('--vh', `${vh}px`);

window.setTimeout(() => {
  let vh = window.innerHeight * 0.01;
  document.documentElement.style.setProperty('--vh', `${vh}px`);
}, 1000);

// Resize
window.addEventListener('resize', () => {
  // We execute the same script as before
  let vh = window.innerHeight * 0.01;
  document.documentElement.style.setProperty('--vh', `${vh}px`);
});

/* Vh Calc (clean code) */

const resize = () => {
  // First we get the viewport height and we multiple it by 1% to get a value for a vh unit
  let vh = window.innerHeight * 0.01;
  // Then we set the value in the --vh custom property to the root of the document
  document.documentElement.style.setProperty('--vh', `${vh}px`);
}

window.setTimeout(() => {
  resize()
}, 1000);

// Resize
window.addEventListener('resize', () => {
  // We execute the same script as before
  resize()
});

/* Vh + Vw Calc */

const calc = () => {
  // First we get the viewport height and we multiple it by 1% to get a value for a vh unit
  let vh = window.innerHeight * 0.01;
  // Then we set the value in the --vh custom property to the root of the document
  document.documentElement.style.setProperty('--vh', `${vh}px`);

  let vw = window.innerWidth * 0.01;
  // Then we set the value in the --vh custom property to the root of the document
  document.documentElement.style.setProperty('--vw', `${vw}px`);
}

window.setTimeout(() => {
  calc()
}, 1000);

// Resize
window.addEventListener('resize', () => {
  // We execute the same script as before
  calc()
});

Wouldn't be better if you set a document ready event listener instead of set timeout just like this:

ES5

['DOMContentLoaded', 'resize'].forEach(function(event) {
  window.addEventListener(event, function() {
    var vh = window.innerHeight * 0.01;
    document.documentElement.style.setProperty('--vh', `${vh}px`);
  });
});

ES6

['DOMContentLoaded', 'resize'].forEach(event => {
    window.addEventListener(event, _ => {
      const vh = window.innerHeight * 0.01;
      document.documentElement.style.setProperty('--vh', `${vh}px`);
    });
 });

Thanks, Youness. Good point! 😉