how to check what time does function take to execute 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 measure function run time

var startTime = performance.now();

alert('Run some function here');

var endTime = performance.now();
var totalTime=endTime-startTime;// time took to run in milliseconds

alert('Total time:'+totalTime +'ms');