What are JavaScript Closures?

A JavaScript Closure is a special type of object that combines a function and the lexical environment within which the function was declared. It allows the function to access and manipulate variables that are external to it. This makes it possible for a function to have “private” variables that are not visible to the outside world. It also makes it possible for the function to remember its state and continue working on it even after it has returned.

How do JavaScript Closures work?

When a function is declared, it captures the variables in its environment, creating a closure. This closure is stored in the function’s [[Environment]] internal property. Whenever the function is invoked, the closure is restored and the function can access all the variables it had access to when it was declared.

To illustrate how closures work, let’s look at a simple example. Suppose you have a function called createCounter that takes in a starting number as an argument and returns a function that increments the counter by one each time it is called:

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function createCounter(startNum) {
  let counter = startNum;

  return function() {
    return ++counter;
  }
}

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In this example, createCounter creates a closure that captures the variable counter. It then returns an anonymous function that has access to the counter variable even after createCounter has returned. When the anonymous function is called, it increments the counter variable and returns its value.

Benefits of JavaScript Closures

Closures are useful for many reasons. They allow us to create functions that can have private variables, which prevents the variables from being accessed and modified by the outside world. They also allow us to create inner functions that can access and manipulate variables from the outer functions, even after the outer functions have returned.

Closures are also useful for creating functions that can remember their state. This makes it possible to create functions that can be called multiple times and remember their previous state. This is often used in event handlers to keep track of the state of the application.

Conclusion

JavaScript Closures are an important tool for creating powerful and flexible functions. They allow us to create functions that have private variables, remember their state, and access variables from their parent functions. With these powerful features, we can create functions that are more robust and easier to maintain.