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

Need further explaination

Since const now = Date.now() and {previousTime: now}, my understanding is that (now - this.state.previousTime) will be equal to zero.

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

    handleStopwatch = () => {
        this.setState( prevState => ({
            isRunning: !prevState.isRunning
        if (!this.state.isRunning) {
            this.setState({ previousTime: Date.now() });

Alright, thanks alot, you have answered my question precisely

1 Answer

Blake Larson
Blake Larson
12,990 Points

setState won't be called until both properties are set so this.state.previousTime is still the old value, but now -prevState.previousTime might be the safer way to do this.