Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial

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
13,014 Points
Blake Larson
Blake Larson
13,014 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.