console.time() starts a timer you can use to track how long an operation takes. You give each timer a unique name, and may have up to 10,000 timers running on a given page. When you call console.timeEnd() with the same name, the browser will output the time, in milliseconds, that elapsed since the timer was started. These functions are not always available in workers. For example, in Firefox, they are available from version 38. More on using the console on mobile devices, see here.