What does the return value of a function declared with the function keyword default to if not specified?

When a function in JavaScript is declared using the function keyword and does not explicitly return a value, the return value defaults to undefined. This means that if the function executes without encountering a return statement, the JavaScript engine will automatically return undefined for that function call.

This behavior is fundamental to understanding how functions operate in JavaScript, as it indicates that a function may complete its operations without giving back a specific result. The idea of an unspecified return value being undefined allows developers to utilize functions that perform actions (like logging or modifying data) without needing to provide a return value.

Thus, if you were to call a function that lacks a return statement, you would receive undefined, which is a distinct and crucial aspect of JavaScript's handling of functions. This contrasts with other potential options, which represent specific values that do not occur as default return values in this context.