It returns the value of time (of type double) in milliseconds. The returned value represents the time elapsed since the execution started.
let t = performance.now();
The code below will give you a brief idea of how this code performs.
Output: In the above code, variables t0 and t1 are used to store starting time and ending time respectively, and while printing we subtracted t0 from t1 to print the time required in the execution of the code.
0 1 2 3 4 5 6 7 8 9 "Call to doSomething took 1.7899998929351568 milliseconds."
0 1 2 3 4 "Call to doSomething took 0.7100000511854887 milliseconds."