Why Does Omitting \'var\' in JavaScript Lead to Unexpected Global Variables and Errors?

The Perils of Global Variable Declarations

In the realm of JavaScript, developers have long been cautioned about a pitfall known as "The Horror of Implicit Globals." This arises when a variable is declared without the "var" keyword.

Contrary to popular belief, omitting "var" does not confer any significant performance benefits. Instead, it creates a global variable, which can lead to unintentional variable leakage and potential conflicts with other scripts on the page.

To illustrate this issue, consider the following function:

<code class="javascript">function foo() {
    variable1 = 5;
    varaible2 = 6;
    return variable1 + variable2;
}</code>

Due to a typo in the declaration of "varaible2," the function returns NaN instead of the expected value of 11. Moreover, a global variable named "varaible2" is inadvertently created, which can be accessed from outside the function:

<code class="javascript">console.log(foo());     // NaN
console.log(varaible2); // 6?!?!?!</code>

This unexpected behavior can lead to confusing and difficult-to-debug errors in your code. To avoid these pitfalls, it is crucial to always use the "var" keyword when declaring variables, both inside and outside functions.

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