Global Variable Is Logged As Undefined When Passed As Parameter To Settimeout Callback Function

When you're working with JavaScript and encounter the situation where a global variable is logged as undefined when passed as a parameter to a setTimeout callback function, it can be a bit confusing at first. But fear not, as we're here to walk you through this scenario and help you understand why this happens and how to address it.

## Understanding the Issue

In JavaScript, when you pass a variable to a `setTimeout` function, the variable is evaluated at the time the function is called, not when it is executed. This can lead to unexpected behaviors, especially when dealing with global variables.

When a global variable is passed as a parameter to a setTimeout callback function, there is a chance that the variable might not hold the expected value when the callback function is executed. Instead, it may appear as `undefined`.

## Why Does This Happen?

The reason behind this behavior is related to the event loop and how JavaScript handles variable scoping. Since the setTimeout callback function is executed asynchronously, by the time it runs, the context of the global variable might have changed or the variable itself may have been modified.

This can create a situation where the passed global variable is no longer pointing to the same value it had when it was initially passed to the setTimeout function.

## How to Resolve the Issue

To address this issue and ensure that the global variable maintains its intended value when passed as a parameter to the setTimeout callback function, you can use a technique called closure.

By creating a closure around the setTimeout function, you can capture the current value of the global variable and ensure that it is preserved when the callback function is executed.

Here's an example of how you can use closure to solve this problem:

Javascript

// Define a global variable
let globalVariable = 'Hello, world!';

(function() {
  // Create a closure around the global variable
  let localCopy = globalVariable;

  // Pass the local copy as a parameter to setTimeout
  setTimeout(function() {
    console.log(localCopy); // 'Hello, world!'
  }, 1000);
})();

By encapsulating the global variable within a closure, you can guarantee that the value remains consistent when the setTimeout callback function is invoked, avoiding the issue of it being logged as `undefined`.

## Conclusion

In conclusion, when you encounter a situation where a global variable is logged as `undefined` when passed as a parameter to a setTimeout callback function, remember that JavaScript's asynchronous nature and variable scoping rules play a significant role in this behavior.

Utilizing closure can help you maintain the expected value of the global variable and ensure that your code behaves as intended in such scenarios. Keep this technique in mind next time you face a similar issue, and happy coding!

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