how to make CSS width property equal to the result of a mathematical expression

Question

I'm trying to make the width property equal to (100/3)% because I am trying to divide the width evenly amongst 3 columns. But obviously this is not valid css. What is the proper way to accomplish this?

Answer 1

pure CSS method:

th, td {
  width: calc(100% / 7);
}

Javascript method: You can get your table elements and assigns styles, try this:

let th = document.getElementsByTagName('th');
let td = document.getElementsByTagName('td');

console.log(th, td)

for(let el of th){
    el.style.width = (100 / 3) + 'px';
}

for(let el of td){
    el.style.width = (100 / 3) + 'px';
}

<table>
  <tr>
    <th>Firstname</th>
    <th>Lastname</th>
    <th>Age</th>
  </tr>
  <tr>
    <td>Jill</td>
    <td>Smith</td>
    <td>50</td>
  </tr>
  <tr>
    <td>Eve</td>
    <td>Jackson</td>
    <td>94</td>
  </tr>
</table>