po-filtered.js

!(function() {
    const nativePO = window.PerformanceObserver
    const polyfills = {
      longtask: LongTaskPolyFill
      // etc.
    }
    window.PerformanceObserver = (callback) => {
      let po, registeredPolyFills = []

      const observe = function(options) {
        po = new nativePO((entries, observer) => {
          callback(entries, this)
        })
        const filteredEntryTypes = po.observe(options)
        (options.entryTypes || []).forEach((entryType) => {
          if ((filteredEntryTypes || []).indexOf(entryType) === -1) {
            const polyfill = polyfills[entryType]
            polyfill && registeredPolyFills.push(polyfill.start(callback))
          }
        })

        if (!filteredEntryTypes.length) {
          po.disconnect()
          po = undefined
        }
        return filteredEntryTypes
      }
      const disconnect = () => {
        if (po) {
          po.disconnect()
          po = undefined
        }

        registeredPolyFills.forEach(p => p.stop())
        registeredPolyFills = []
      }

      return {
        observe,
        disconnect
      }
    }
})()