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

Brian Hoang
Brian Hoang
11,515 Points

setState in handleStopwatch()

The fact that in the video "!this.state.isRunning" is evaluated to "true" and console logs "starting..." the first time Guil clicks the start button implies that the isRunning state was not changed by setState before the "if(!this.state.isRunning)" line

Should I from now on assume the effect of setState method is not immediate, at least in the function it was called?

1 Answer

Zaberca David
Zaberca David
2,965 Points

This is the same thing I was wondering about. At 4:33, the instructor says 'only when starting the timer'. In concept, when the timer starts, the isRunning flag goes from false, to true. The if statement condition here is (!this.state.isRunning), which goes true when isRunning is set to false. That means the previous setState method did not change the state.

Can somebody confirm this and maybe explain further or give some references? Thank you

Zaberca David
Zaberca David
2,965 Points

Ok, so I researched further and found the answer. It seems that since setState is an async function, and async function are put into event loop and always executed after the call stack is cleared, it is guaranteed that the this.state.isRunning will be false for the entirety of the handleStopWatch.