Function Name Scope

As you’ve seen by now, a function declaration looks like this:

function askQuestion() {
    // ..
}

And as discussed in Chapters 1 and 2, such a function declaration will create an identifier in the enclosing scope (in this case, the global scope) named askQuestion.

What about this program?

var askQuestion = function(){
    // ..
};

The same is true for the variable askQuestion being created. But since it’s a function expression—a function definition used as value instead of a standalone declaration—the function itself will not “hoist” (see Chapter 5).

One major difference between function declarations and function expressions is what happens to the name identifier of the function. Consider a named function expression:

var askQuestion = function ofTheTeacher(){
    // ..
};

We know askQuestion ends up in the outer scope. But what about the ofTheTeacher identifier? For formal function declarations, the name identifier ends up in the outer/enclosing scope, so it may be reasonable to assume that’s the case here. But ofTheTeacher is declared as an identifier inside the function itself:

var askQuestion = function ofTheTeacher() {
    console.log(ofTheTeacher);
};
askQuestion();
// function ofTheTeacher()...
console.log(ofTheTeacher);
// ReferenceError: ofTheTeacher is not defined

Not only is ofTheTeacher declared inside the function rather than outside, but it’s also defined as read-only:

var askQuestion = function ofTheTeacher() {
    "use strict";
    ofTheTeacher = 42;   // TypeError
    //..
};
askQuestion();
// TypeError

Because we used strict-mode, the assignment failure is reported as a TypeError; in non-strict-mode, such an assignment fails silently with no exception.

What about when a function expression has no name identifier?

var askQuestion = function(){
   // ..
};

A function expression with a name identifier is referred to as a “named function expression,” but one without a name identifier is referred to as an “anonymous function expression.” Anonymous function expressions clearly have no name identifier that affects either scope.