index.js

const defaultOptions = { display: 'swap', replace: false }

function optionsByFont (rule, options) {
  let font = null
  let result = defaultOptions
  rule.walkDecls('font-family', decl => {
    font = decl.value
    return false
  })
  if (font) {
    options.forEach(opt => {
      if (opt.test) {
        let pattern = new RegExp(opt.test)
        if (pattern.test(font)) result = opt
      } else {
        result = opt
      }
    })
  }
  return result
}

/**
 * @type {import('postcss').PluginCreator}
 */
module.exports = (opts = {}) => {
  let options = opts || defaultOptions

  if (options && !Array.isArray(options)) {
    options = [options]
  }

  return {
    postcssPlugin: 'postcss-font-display',
    Root (root) {
      root.walkAtRules('font-face', rule => {
        let exists = false
        let fontOptions = optionsByFont(rule, options)
     
        rule.walkDecls('font-display', decl => {
          if (fontOptions.replace) {
            decl.value = fontOptions.display
          }
          exists = true
        })
     
        if (!exists) {
          rule.append({ prop: 'font-display', value: fontOptions.display })
        }
      })
    }
  }
}

module.exports.postcss = true