What are the drawbacks of using global variables?

Is There a Performance Impact to Using Global Variables in JavaScript?
Why is it bad to make elements global variables in Javascript?

The concept of global variables in JavaScript has long been frowned upon, likened to using street slang compared to more structured languages like Java or C++. But what are the actual consequences of utilizing global variables? Does it impact performance or potentially lead to browser crashes? Are there noticeable differences when relying on globals?

While adhering to best practices and expert advice, understanding the rationale behind avoiding global variables can provide valuable insight into the workings of JavaScript. Learning this language independently highlights the importance of grasping fundamental principles - why exactly do experts caution against widespread use of global variables?

javascriptglobal-variables

Answer №1

Using global variables can lead to unforeseen effects (known as action at a distance).

For instance, if you intend to utilize a local variable i, but forget to declare it with var, and if some other code (that executed earlier) also used the same variable name, then you will begin using the old value of that variable. This can cause your code to behave unpredictably and may be difficult to troubleshoot ("How did that value end up here?").

Just recently, a similar issue was discussed in a question on Java on Stack Overflow (link) which resulted in significant confusion.

Furthermore, using global variables extends the scope of the variable, meaning its memory cannot be released even when not needed anymore, potentially leading to memory leaks.

Answer №2

When approaching programming with a well-organized strategy, utilizing global variables can be deemed as appropriate and even beneficial in terms of cutting down on unnecessary overhead caused by continual passing and copying of pointers and variables.

In my opinion, the decision to use local variables is primarily based on how it enhances the code's manageability, debuggability, and compatibility with different systems.

Answer №3

There are a couple of key issues that need to be addressed in this situation:

  • The problem of hoisting
  • The risk of redefining external variables

The hoisting problem arises when attempting to declare the same variable within what may appear to be different scopes, but are actually not (such as inside and outside the braces of an if statement).

In JavaScript, all variable declarations are moved to the top before execution and then values are assigned where the variable was originally declared and assigned. This can lead to unexpected bugs where you unintentionally redefine your own global variables.

Another issue is that when utilizing external libraries in your project, there is a possibility of overriding an existing variable, potentially resulting in loss of functionality and introduction of subtle bugs.

I would suggest prioritizing local variables whenever possible, or utilizing "namespaces" to keep them separate.

Adding to the complexity, JavaScript does not provide warnings for either of these issues, making them challenging to detect.

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