The workerStart read-only property of the PerformanceResourceTiming interface returns a DOMHighResTimeStamp immediately before dispatching the FetchEvent if a Service Worker thread is already running, or immediately before starting the Service Worker thread if it is not already running. If the resource is not intercepted by a Service Worker the property will always return 0.




A DOMHighResTimeStamp.


In the following example, the value of the *Start and *End properties of all "resource" type events are logged.

function print_PerformanceEntries() {
  // Use getEntriesByType() to just get the "resource" events
  var p = performance.getEntriesByType("resource");
  for (var i=0; i < p.length; i++) {
function print_start_and_end_properties(perfEntry) {
  // Print timestamps of the PerformanceEntry *start and *end properties 
  properties = ["connectStart", "connectEnd",
                "domainLookupStart", "domainLookupEnd",
                "redirectStart", "redirectEnd",
                "responseStart", "responseEnd",

  for (var i=0; i < properties.length; i++) {
    // check each property
    var supported = properties[i] in perfEntry;
    if (supported) {
      var value = perfEntry[properties[i]];
      console.log("... " + properties[i] + " = " + value);
    } else {
      console.log("... " + properties[i] + " = NOT supported");


Specification Status Comment
Resource Timing Level 1
The definition of 'workerStart' in that specification.
Candidate Recommendation Initial definition.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support. 43 (Yes) 58 (58) No support 32 No support
Feature Android Edge Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support. 4.4 (Yes) 58.0 (58) No support 32 No support

Document Tags and Contributors

 Contributors to this page: chrisdavidmills, libbymc, rolfedh, AFBarstow
 Last updated by: chrisdavidmills,