Why Does Variable Declaration at Console Logs \'Undefined\'?

Unveiling the Mystery: Why Variable Declaration at Console Logs "Undefined"

In the realm of JavaScript, a peculiar phenomenon arises when declaring variables at the console. Instead of the expected value, it prints "undefined." This puzzling behavior has baffled many developers, leaving them seeking a comprehensive explanation.

While previous Stack Overflow discussions have addressed this issue, they fail to delve into the root cause. To fill this void, let's delve into the depths of JavaScript's evaluation process to uncover the truth behind this enigmatic behavior.

Contrary to popular belief, the declaration of a variable, such as var a;, is considered a valid expression by JavaScript. Therefore, it evaluates to a value, in this case, undefined. However, the real surprise lies not in the declaration itself but in the fact that the console prefers to display the result of this expression instead of the variable's eventual value.

This seemingly counterintuitive behavior extends to function declarations as well. When function anyFunctionName() {} is entered into the console, it also prints undefined. The reason for this is nestled within the intricacies of eval statements.

According to the ECMAScript specification, "the result of evaluating the program prog" is returned as (normal, empty, empty) unless the completion value is a non-empty value. In the case of variable and function declarations, the completion value is empty. This means that the console receives (normal, empty, empty) for both these statements.

However, the crux of the matter lies in the evaluation of multiple SourceElements. When a script contains multiple SourceElements, the result of evaluating each element is combined. If the result is (normal, empty, empty), the console falls back to the result of the preceding SourceElement with a non-empty completion value.

This intricate evaluation process explains the seemingly strange behavior of your code. In the example provided, var a = 3; a = 4; returns 4 because the assignment statement a = 4 has a non-empty completion value.

In essence, the console's behavior in this context is a reflection of the intricacies of JavaScript's evaluation process. Understanding these internal mechanisms empowers you to navigate the JavaScript landscape with confidence and clarity.

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