how to calculate function execution time in javascript code example

Example 1: time taken for a function javascript

console.time("timer");   //start time with name = timer
console.timeEnd("timer"); //end timer and log time difference

Example 2: javascript time execution

var t0 = performance.now();

doSomething();   // <---- The function you're measuring time for 

var t1 = performance.now();
console.log("Call to doSomething took " + (t1 - t0) + " milliseconds.");