index.js

const { library, dom, icon: getIcon } = require('@fortawesome/fontawesome-svg-core')

function tryInstall(block) {
  try {
    var icons = block()
    library.add(icons)
    return true
  } catch(ex) {
    return false
  }
}

tryInstall(function () { return require('@fortawesome/free-solid-svg-icons').fas })
tryInstall(function () { return require('@fortawesome/free-regular-svg-icons').far })
tryInstall(function () { return require('@fortawesome/free-brands-svg-icons').fab })

function faCss() {
  return dom.css()
}

function faInline(iconName, opts) {
  var options = opts || {prefix: 'fas'}
  var prefix = options.prefix

  var icon = getIcon({ prefix: prefix, iconName: iconName })
  if (!icon) {
    throw new Error(
      'Can not find icon "' + iconName +'" with prefix "' + prefix + '"' +
      'Make sure you have installed also a corresponding icons package:\n' +
      ' - @fortawesome/free-solid-svg-icons for fas prefix \n' +
      ' - @fortawesome/free-regular-svg-icons for far prefix\n' +
      ' - @fortawesome/free-brands-svg-icons for fab prefix\n'
    )
  }
  return icon.html
}

hexo.extend.helper.register('fa_css', faCss)
hexo.extend.helper.register('fa_inline', faInline)

hexo.extend.tag.register('fa_css', function () {
  return '<style>' + faCss() + '</style>'
})

hexo.extend.tag.register('fa_inline', function (args) {
  var iconName = args[0]
  var prefix = args[1] || 'fas'
  return faInline(iconName, {prefix: prefix})
})