po-interface-check.js

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

      const observe = function(options) {
        filteredEntryTypes = (options.entryTypes || []).filter((entryType) => {
          const hasNativeSupport = typeof window[interfaces[entryType]] !== 'undefined'
          if (!hasNativeSupport) {
            const polyfill = polyfills[entryType]
            polyfill && registeredPolyFills.push(polyfill.start(callback))
          }
          return hasNativeSupport
        })

        if (filteredEntryTypes.length) {
          po = new nativePO((entries, observer) => {
            callback(entries, this)
          })
          po.observe({
            ...options, 
            entryTypes: filteredEntryTypes,
          })
        }
      }
      const disconnect = () => {
        if (po) {
          po.disconnect()
          po = undefined
        }

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

      return {
        observe,
        disconnect
      }
    }
})()