Copy and paste me:

function measureText(string, fontSize = 10) {
  const widths = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0.5,0.6,0.7,1.3,1,1.5,1.2,0.4,0.6,0.6,0.8,1.3,0.5,0.6,0.5,0.6,1,1,1,1,1,1,1,1,1,1,0.5,0.5,1.3,1.3,1.3,0.9,1.6,1.1,1.1,1.1,1.2,1,0.9,1.2,1.2,0.5,0.6,1.1,0.9,1.4,1.2,1.3,1,1.3,1.1,1,1.1,1.2,1.1,1.6,1.1,1.1,1.1,0.6,0.6,0.6,1.3,1,0.8,1,1,0.9,1,1,0.6,1,1,0.4,0.5,1,0.4,1.6,1,1,1,1,0.7,0.8,0.6,1,0.9,1.3,0.9,0.9,0.8,1,0.5,1,1.3]
  const avg = 0.9536842105263161
  return string
    .split('')
    .map(c => c.charCodeAt(0) < widths.length ? widths[c.charCodeAt(0)] : avg)
    .reduce((cur, acc) => acc + cur) * fontSize
}

Example usage:

measureText('Hello world', 12)
// 106.80000000000001