Why Does JavaScript Console Print \'Undefined\' for Variable Declarations?

Why JavaScript Variable Declaration at Console Prints "Undefined"

When declaring a variable using the var keyword at the console, one might be surprised to see "undefined" printed as the result.

The Cause

In JavaScript, the var keyword declares a variable, but without explicitly assigning it a value, it remains undefined. When a variable declaration statement is evaluated at the console, the result is the value of the expression being evaluated, which, in this case, is the undefined variable.

The Exception

However, it's worth noting that when a variable is declared with an assignment, the result printed is the assigned value. For example, var a = 5 will print 5.

The Underlying Complexity

The behavior of the console when dealing with var declarations can be traced back to the semantics of the JavaScript evaluation model. According to the ECMAScript specification:

• The var statement returns a completion value of (normal, empty, empty).
• The result of an expression evaluated as part of a SourceElement is returned unless that result is (normal, empty, empty).

In the case of var a;, since there's no assignment, the completion value is (normal, empty, empty). Therefore, the evaluation result is undefined.

The Hidden Surprise

Another interesting observation is that function declarations also return (normal, empty, empty) and thus print undefined when evaluated at the console. However, when a function is declared as an expression, such as (function f() {}), the result is the function itself. This distinction highlights the subtle difference between Function Declarations and Function Expressions in JavaScript.

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