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jun cheng wong
17,021 PointsNeed 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() });
}
};
1 Answer

Blake Larson
12,990 PointssetState
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.
jun cheng wong
17,021 Pointsjun cheng wong
17,021 PointsAlright, thanks alot, you have answered my question precisely