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

Posted by jun cheng wong
jun cheng wong
17,021 Points

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() });
        }
    };
jun cheng wong
jun cheng wong
17,021 Points

Alright, thanks alot, you have answered my question precisely

1 Answer

Blake Larson
Blake Larson
12,990 Points
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.