Are Implicit Global Variable Declarations in JavaScript Functions Beneficial?

Declaring Global Variables Inside Functions

In JavaScript, variables can be declared either implicitly by omitting the var keyword or explicitly by using it. While W3Schools suggests that implicit declarations create global variables, is there any practical benefit to doing so within functions?

No Benefit for RAM or Performance

Contrary to popular belief, declaring global variables implicitly offers no advantage in terms of memory usage or execution speed.

The Horror of Implicit Globals

The issue with implicit global variables lies in their vulnerability to typos and accidental overwrites. If a variable is declared implicitly inside a function, it becomes accessible globally. This can lead to unexpected behaviors and errors, especially when typos occur in variable names.

For instance, consider the following function:

function foo() {
    var variable1, variable2;

    variable1 = 5;
    varaible2 = 6;
    return variable1 + variable2;
}

Due to a typo in the varaible2 declaration, the function will return NaN instead of 11. Moreover, the misspelled variable will become a global variable, potentially causing conflicts elsewhere in the codebase.

Explicit Declarations for Clarity and Control

To avoid such pitfalls, it's highly recommended to explicitly declare variables using the var keyword, whether inside or outside of functions. This ensures that variables are properly scoped, prevents typos from becoming global problems, and enhances code readability.

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