Function expressions vs function declarations: return value

Question

In a Udacity lesson the difference between function expressions and declarations is explained as follows:

A function declaration defines a function and does not require a variable to be assigned to it. It simply declares a function, and doesn't itself return a value ... On the other hand, a function expression does return a value.

This is confusing; to my knowledge, when both function expressions and function declarations include a return statement, both return a value.

If I understand correctly, the difference with respect to return value is that in a function expression, if a value is changed in the first call of the function, in a subsequent call the updated value would be preserved—whereas if the function were not stored in a variable, the return value would be erased when the function is finished executing. Am I missing something, and is the statement from the lesson accurate?

Note: My question is different from the one marked as a duplicate. In that question it asks what the reasons for using one over the other, but return value is not mentioned in the question or explained in its answers.

Answer 1

To understand what this really is about, we need to dig into the JavaScript grammar:

In ECMAScript a script consists of Statements and Declarations. A Statement can be (amongst others) an ExpressionStatement. Note that ExpressionStatement is explicitly defined as:

ExpressionStatement[Yield, Await]:

[lookahead ∉ { {, function, async [no LineTerminator here] function, class, let [ }]

Expression[+In, ?Yield, ?Await];

This looks really cumbersome, but what it says is that an ExpressionStatement cannot possibly start with the keyword function. So if you just write:

function a() {}

This can never be interpreted as an expression although in other circumstances like

const a = function a() {}

it is an expression. (The right hand side of an assignment operation always must be an expression.)

Now, only expressions evaluate a value, statements do not. This is all the text you quote is saying in a hard to understand way.

A function declaration defines a function and does not require a variable to be assigned to it:

True but redundant. A declaration cannot occur at the right hand-side of an assignment.

It simply declares a function, and doesn't itself return a value ...

Yeah, statements do not evaluate to ("return") a value.

On the other hand, a function expression does return a value.

Sure, like any expression.

---

See https://www.ecma-international.org/ecma-262/8.0/#prod-StatementList

Answer 2

The definition isn't talking about the function returning a value, it is talking on how one way of creating a function returns a value (which is the function expression) and another just declares a function (a function declaration).

To help you clarify things, you should understand first what an expression is:

An expression is any valid unit of code that resolves to a value.

One example of an expression would be x = 5, which evaluates to 5. Another example would be 3 + 2, which also evaluates to 5.

In other words, we could say that both expressions above return a value of 5.

A function expression is just an expression that returns (evaluates to) a function:

// expression
const whatever = function expression() {}
//               ^^^^^^^^^^^^^^^^^^^^^^^^ this is the function expression in an assignment statement

A function declaration is not an expression but a statement. It doesn't need to evaluate to a value. It is just immediately declared:

// declaration
function example() {}

How a function is created (via a declaration or an expression) doesn't affect what the function can return - that capability is the same in both cases.