Simply put, closure is a function that remembers its outer scope (the lexical environment). The state of the outer function is remembered by the inner function and can work with that state at a later time, even though the outer function is not being held by the execution context at any time the state is referenced.
In the above code snippet, the function returned by the sayHello() captures the value for the greet variable and logs onto the console.
The behavior of this code is pretty straightforward, but there can be some odd behaviors when working with closures.
In the above code snippet, createIncrement() function returns an array that contains two function references are increment and log, respectively. Both functions have access to the closure created by their outer function, but calling the log() function doesn't seem to access the updated value for the value variable.
The reason is the message variable never gets executed upon receiving a new value for the value variable and log() function refers to the same old snapshot of the message variable. This can be easily solved by including the message variable itself inside the log function, so whenever it gets executed message has the updated value for the value variable.
A closure that maintains some outdated state is called a stale closure. Stale closures have always been a problem when using React hooks, especially useState() hook and useEffect() hook.
We will see some of the scenarios that this problem can occur, and solutions to prevent some unexpected behaviors in our application caused by stale closures as well.
Issue with the useState() Hook
The App component maintains a count state that will be incremented by one per 3 seconds with the setTimeout() timer function. But if we click on the Increase button multiple times at a fast phase (all the subsequent calls should happen within 3 secs) you will notice that the value is incremented only by one.
This behaves this way because each time the function executes it has access to the old value because the value only gets updated after some seconds, which results in the component not being re-rendered as the state value remains the same on each call.
To prevent this behavior from happening, we can provide a pure function to the setCount() method which is then guaranteed to be called only after the count value has been updated (the component has been re-rendered with the new value!).
Issue with the useEffect() Hook
In this above code snippet, App component has the useEffect() function which calls the timer function setInterval once the component has been mounted. You are free to change the state after it has been mounted and the count value gets incremented by one. But the issue here is that the setInterval() function still has the snapshot of the old value for the count variable which was initialized from useState() function.
To fix this issue, we can pass the count variable into the dependency array, which causes the useState() function to execute again whenever the value for the count variable gets changed so that we have the updated value for each setInterval() function call.
So, the important thing to understand is that we always need to make sure to design our hooks inside the functional component which uses the closure which always maintains a fresh state of its data, so that it will avoid falling our app into pretty unexpected behaviors.