JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://rawgit.com/schickling/calculate-size/master/src/calculate-size.js"></script>
<span>Hello World</span>

<br>
<br>
Check my size with dev tools

CSS

span {
    font-family: Arial;
    font-size: 12px;
}

JavaScript

var size = calculateSize("Hello world!", {
   font: 'Arial',
   fontSize: '12px'
});

console.log(size.width); // 65
console.log(size.height); // 14