What is the difference between a return value of undefined and a situation where a function does not specify a return operator?

In JavaScript, if a function does not contain a return statement, it returns undefined by default. That is, the absence of a return statement and an explicit return undefined are effectively equivalent in terms of the return value.

Example:

function foo() {}
function bar() { return undefined; }

console.log(foo()); // undefined
console.log(bar()); // undefined

Thus, there is no difference between a function without a return and a function with return undefined — both return undefined. The only difference might be in the developer's intent when they explicitly specify return undefined.