Why Is \'Undefined\' Displayed for Variable Declarations in the JavaScript Console?

Why is "undefined" Printed When Declaring Variables in the JavaScript Console?

Unlike in other environments, declaring variables in the console using the "var" keyword does not result in the variable being assigned a value. Instead, the console prints "undefined."

Understanding the Expression

Contrary to what it may seem, "var a;" is a valid expression that evaluates to "undefined." This is because declaring a variable without assigning it a value effectively creates an uninitialized variable, which has a default value of "undefined."

The Behavior of Eval

The JavaScript console uses the "eval" function to evaluate expressions. According to the eval specification, if an expression evaluates to an empty value (such as an uninitialized variable), "undefined" is returned.

Impact on Other Statements

Interestingly, this behavior extends to other "var" and function declarations. If any subsequent statement has a "real" result, the console will ignore the "var" and function declarations and display the result of the other statement. For example:

> var a = 3; var a = 4; var a = 5; function f() {};
4 // !!!

Why the Exception with Assignments

In the case of assignments ("var a = 4;"), the behavior changes because the assignment statement has a non-empty result (the value assigned). This result overrides the empty result of the preceding "var" declaration.

Function Declarations vs. Expressions

While both function declarations and expressions use the "var" keyword, they are evaluated differently. Function declarations (e.g., "function f() {}") evaluate to "undefined" (similar to variable declarations), while function expressions (e.g., "(function f() {})") evaluate to the function itself.

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