指定したフォントが存在するか真偽値を返す関数。
function font_exists(fontName){
const canvas = document.createElement('canvas').getContext('2d')
const text = 'abcdefghijklmnopqrstuvwxyz0123456789'
canvas.font = `72px monospace`
const size1 = canvas.measureText(text).width
canvas.font = `72px ${fontName}, monospace`
const size2 = canvas.measureText(text).width
return size1 !== size2
}