Does assigning to a function overwrite the function or create an implicit global?

In JavaScript we can declare a function and then assign to it, as follows:

function spam() { return "spam 'n eggs"; }
spam = spam();
spam(); // TypeError: spam is not a function.

Does this code change the value of a global variable called spam, or create an implicit global variable that shadows the function spam?

It doesn't overwrite the function, but it does assign a string to that variable. The function defined on the first line returns a string, and because the second line sets spam equal to the return value (since the function is called (notice the parentheses)) spam then is a string.

Try this in the browser console: window.spam. After the first line, it should reveal that function. Then after the second line it should show that string.

function spam() {
  return "spam 'n eggs";
}
console.log('typeof spam: ',typeof spam, 'typeof window.spam: ',typeof window.spam);
spam = spam(); //spam is assigned a string
console.log('typeof spam: ',typeof spam, 'typeof window.spam: ',typeof window.spam);
spam(); // TypeError: spam is not a function.

if we moved that code inside a function, the var keyword could be used for local scope:

function assignSpam() {
        var spam = function() { return "spam 'n eggs"; }
        console.log('typeof spam after initial assignment: ',typeof spam);
        spam = spam(); //spam is assigned a string
        console.log('typeof spam after assigning to spam(): ',typeof spam);
        spam(); // TypeError: spam is not a function.
    }
    assignSpam();
    spam; //undefined

Results should be similar in server-side javascript (e.g. NodeJS).