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JavaScript React Components (2018) Stateful Components and Lifecycle Methods Update the Stopwatch State with componentDidMount()

Can someone please explain the concept behind elapsedTime and previousTime?


1 Answer

Say your current time is 12:00, then in 1 minute, your current time is 12:01. When it is 12:01 you can get the elapsed time by subtracting 12:00 from 12:01 which would be 1 minute. You then add that 1 minute to the elapsed time variable which starts at zero. At 12:02 you would subtract 12:01 (Which would be the previous time) to get another minute of elapsed time which you should then add to the elapsed time variable. At this point, you will have 2 minutes in the elapsed time variable.

We need to do things this way because the setInterval is not precise but you can get a precise time with Date.now()