How to fix the “cannot read properties of undefined (reading ‘style’)” error with JavaScript?

To fix the "cannot read properties of undefined (reading ‘style’)" error with JavaScript, we should make sure the element we’re selecting isn’t null.

For instance, we write

const boxElement = document.querySelector(".box");
if (boxElement) {
  boxElement.style.width = "100px";
  boxElement.style.height = "100px";
  boxElement.style.backgroundColor = "#f00";
}

to select the element with the class box with querySelector.

Then if boxElement isn’t null, then we set its width, height and backgroundColor styles to apply the style values we want.

If the element isn’t null, then the style property should always be an object.