Is this example in the book really a closure or just global scope reference?

[akasuv]

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Edition: 2nd

Book Title: Scope & Closures

Chapter: Chapter 7: Using Closures

Section Title: Live Link, Not a Snapshot

Question: variable studentName and function greeting() are both in the global scope, so when greeting() invoked, is studentName in greeting a closure reference or just global scope lookup?

Example in the book:

var studentName = "Frank";

var greeting = function hello() {
    // we are closing over `studentName`,
    // not "Frank"
    console.log(
        `Hello, ${ studentName }!`
    );
}

// later

studentName = "Suzy";

// later

greeting();
// Hello, Suzy!

[getify]

Both.

[akasuv]

Hi, @getify, thanks for your reply! I can understand why it's a global scope lookup, but the closure part seems a little bit confusing, like you said in the book, one of conditions being a closure is:

Must be invoked in a different branch of the scope chain from the variable(s)

studentName and greeting are in the same scope branch which is global, and greeting() is also invoked on global scope, isn't it just a normal global scope lookup?

[getify]

Here's the relevant perspective to consider:

1. If I were to take that snippet and do nothing else to it but wrap it in an outer scope (block, function, or module)... wouldn't we say that nothing had changed about its behavior? And if nothing changed about its behavior, isn't it fair to say that its nature (the how of its workings) is also the same (in spirit, if not in fact)?

   Such an alternate version of that code being contemplated is unquestionably closure. So the global-scope-only version is, in spirit, also closure. It's not terribly helpful to create this exception in your mind where code in the global scope cannot be closure when if the exact same code appeared elsewhere, it would be closure.

2. The example in question is a very narrow example to illustrate an isolated point. I could have gone to the trouble of wrapping an outer scope around it to force a closure context... but what's the point of that? It creates unnecessary noise for no real benefit.

3. Moreover, mathematically, it's a closure. IOW, in theory, it's a closure. In practice, the engines may or may not use the same mechanism or an exception path. Who knows?

It is true that in my courses and books, I do argue that closure which isn't observably different from the absence of closure is not really closure. IOW, theoretical but unobservable expressions of closure aren't terribly useful to us programmers; they're academic trivia.

While I stand by that in general, that's not really as much a claim about identity; rather it's more a claim about utility. An example of closure that's indistinguishable from global scope is not a particularly useful closure example. But that's not as strong a claim as "this is absolutely not closure in any way."

The example in question is a weak/useless example of closure, but in theory and spirit, if not practice, it's still a closure, as opposed to the contrary absolute claim of it not being a closure at all -- a claim I'm not willing to make.

[perioad]

Hello @getify :)
Could you please illustrate with code your point? It would be so helpful!

If I were to take that snippet and do nothing else to it but wrap it in an outer scope (block, function, or module)

[getify]

@perioad

This quoted code above:

var studentName = "Frank";

var greeting = function hello() {
    // we are closing over `studentName`,
    // not "Frank"
    console.log(
        `Hello, ${ studentName }!`
    );
}

// later

studentName = "Suzy";

// later

greeting();
// Hello, Suzy!

...becomes this code with an outer scope wrapped around it (via an IIFE):

(function OuterScopeIIFE(){

   var studentName = "Frank";

   var greeting = function hello() {
       // we are closing over `studentName`,
       // not "Frank"
       console.log(
           `Hello, ${ studentName }!`
       );
   }

  // later

   studentName = "Suzy";

   // later

   greeting();
  // Hello, Suzy!

})();

[perioad]

@getify
Thanks for finding the time and answering my question! It makes much more sense for me now. Cheers!

[haoyi2015]

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