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

Posted by Shung Chen
Shung Chen
6,526 Points

Confusion in getting true elapsedTime

    tick = () =>{
        if (this.state.isRunning){
            const now = Date.now();
            this.setState((prevState)=>({
                previousTime: now,
                elapsedTime: prevState.elapsedTime + (now - 
                this.state.previousTime)
            }));
        }
    }

when the function is triggered, isn't 

  elapsedTime: prevState.elapsedTime + (now - 
                this.state.previousTime)

becomes

  elapsedTime: prevState.elapsedTime + (this.state.previousTime - 
                this.state.previousTime)

And hence they are pointing to the same reference, the elapsed time will always be 0?