Definition of 'closures'

Question

Let me ask one question. It's about closures in JavaScript, but not about how they work.

David Flanagan in his "JavaScript The Definitive Guide 6th Edition" wrote:

...Technically, all JavaScript functions are closures: they are objects, and they have a scope chain associated with them....

Is this correct? Can I call every function (function object + it's scope) a "closure"?

And stacks' tag "closures" says:

A closure is a first-class function that refers to (closes over) variables from the scope in which it was defined. If the closure still exists after its defining scope ends, the variables it closes over will continue to exist as well.

In JavaScript every function refers to variables from the scope in which it was defined. So, It's still valid.

The question is: why do so many developers think otherwise? Is there something wrong with this theory? Can't it be used as general definition?

Answer 1

Technically, all functions are closures. But if the function doesn't reference any free variables, the environment of the closure is empty. The distinction between function and closure is only interesting if there are closed variables that need to be saved along with the function code. So it's common to refer to functions that don't access any free variables as functions, and those that do as closures, so that you know about this distinction.

Answer 2

It's a tricky term to pin down. A function that's simply declared is just a function. What makes a closure is calling the function. By calling a function, space is allocated for the parameters passed and for local variables declared.

If a function simply returns some value, and that value is just something simple (like, nothing at all, or just a number or a string), then the closure goes away and there's really nothing interesting about it. However, if some references to parameters or local variables (which, mostly, are the same) "escape" the function, then the closure — that space allocated for local variables, along with the chain of parent spaces — sticks around.

Here's a way that some references could "escape" from a function:

function escape(x, y) {
  return {
    x: x,
    y: y,
    sum: function() { return x + y; }
  };
}

var foo = escape(10, 20);
alert(foo.sum()); // 30

That object returned from the function and saved in "foo" will maintain references to those two parameters. Here's a more interesting example:

function counter(start, increment) {
  var current = start;
  return function() {
    var returnValue = current;
    current += increment;
    return returnValue;
  };
}

var evenNumbers = counter(0, 2);
alert(evenNumbers()); // 0
alert(evenNumbers()); // 2
alert(evenNumbers()); // 4

In that one, the returned value is itself a function. That function involves code that makes reference to the parameter "increment" and a local variable, "current".

I would take some issue with conflating the concept of a closure and the concept of functions being first-class objects. Those two things really are separate, though they're synergistic.

As a caveat, I'm not a formalist by basic personality and I'm really awful with terminology so this should probably be showered with downvotes.

Answer 3

I would try to answer your question knowing you were asked about what closures are during the interview (read it from the comments above).

First, I think you should be more specific with "think otherwise". How exactly?

Probably we can say something about this noop function's closure:

function() {}

But it seems it has no sense since there are no variables would bound on it's scope.

I think even this example is also not very good to consider:

function closureDemo() {
    var localVar = true;
}
closureDemo();

Since its variable would be freed as there is no possibility to access it after this function call, so there is no difference between JavaScript and let's say C language.

Once again, since you said you have asked about what closures are on the interview, I suppose it would be much better to show the example where you can access some local variables via an external function you get after closureDemo() call, first. Like

function closureDemo() {
    var localVar = true;
    window.externalFunc = function() {
        localVar = !localVar;  // this local variable is still alive
        console.log(localVar); // despite function has been already run,
                               // that is it was closed over the scope
    }
}
closureDemo();
externalFunc();
externalFunc();

Then to comment about other cases and then derive the most common definition as it more likely to get the interviewer to agree with you rather than to quote Flanagan and instantly try to find the page where you've read it as a better proof of your statement or something. Probably your interviewer just thought you don't actually know about what closures are and just read the definition from the book. Anyhow I wish you good luck next time.

Answer 4

The definition is correct. The closure keeps the scope where it was born

Consider this simple code:

getLabelPrinter = function( label) {
  var labelPrinter = function( value) {
      document.write(label+": "+value);
  }
  return labelPrinter; // this returns a function
}

priceLabelPrinter = getLabelPrinter('The price is');
quantityLabelPrinter = getLabelPrinter('The quantity is');

priceLabelPrinter(100);

quantityLabelPrinter(200);

//output:
//The price is: 100 The quantity is: 200  

https://jsfiddle.net/twqgeyuq/