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

Begana Choi
PLUS
Begana Choi
Courses Plus Student 13,125 Points
Posted by Begana Choi
Begana Choi
Courses Plus Student 13,125 Points

why don't we use prevState in tick function for ( now - this.state.previousTime )?

prevState is the better way to verify that state did update correctly. but why we don't use prevState for this part?

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

1 Answer

Mia Filisch
Mia Filisch
15,381 Points
Mia Filisch
Mia Filisch
15,381 Points

I was just wondering the same thing! My understanding is that it's always better to use prevState than this.state when updating state based on existing state, so I assume this is simply an oversight. I used prevState in both places and it seems to work fine!