Function returning a variable without declared it

Question

I recently search in the code of the library of knockout to find how observables are able to create dependencies with computed functions when we call it.

In the source code, I found the function linked to observables creation:

ko.observable = function (initialValue) {
    var _latestValue = initialValue;

    function observable() {
        if (arguments.length > 0) {
            // Write

            // Ignore writes if the value hasn't changed
            if (observable.isDifferent(_latestValue, arguments[0])) {
                observable.valueWillMutate();
                _latestValue = arguments[0];
                if (DEBUG) observable._latestValue = _latestValue;
                observable.valueHasMutated();
            }
            return this; // Permits chained assignments
        }
        else {
            // Read
            ko.dependencyDetection.registerDependency(observable); // The caller only needs to be notified of changes if they did a "read" operation
            return _latestValue;
        }
    }
    ko.subscribable.call(observable);
    ko.utils.setPrototypeOfOrExtend(observable, ko.observable['fn']);

    if (DEBUG) observable._latestValue = _latestValue;
    observable.peek = function() { return _latestValue };
    observable.valueHasMutated = function () { observable["notifySubscribers"](_latestValue); }
    observable.valueWillMutate = function () { observable["notifySubscribers"](_latestValue, "beforeChange"); }

    ko.exportProperty(observable, 'peek', observable.peek);
    ko.exportProperty(observable, "valueHasMutated", observable.valueHasMutated);
    ko.exportProperty(observable, "valueWillMutate", observable.valueWillMutate);

    return observable;
}

What I think is very weird is the returns of 'observable' where I don't found any declaration of this variable. Sure that great men who created this library don't forget to declared it.

How it is possible to use a variable without declared it and prevent it to be put in a global scope?

My feeling is we can used a function declaration as a variable when this function declaration is declared inside another function but I'm really not sure about how it works.

Edit: After searching on the web, I found this article.

In this article, the guy write this:

Use declarations, please "In the code of unexperienced developers, functions are often declared by expressions:

... code ... var f = function() { ... } ... Function Declarations are much more readable and shorter. Use them instead.

... code ... function f() { ... } ... Besides, functions declared this way can be called before it’s definition.

Use expressions only if you mean it. E.g for conditional function definition."

Ok, Am I an unexperienced developer? I don't think so. I just don't read all the odds of Javascript. :)

The author of the article who said in the code of unexperienced developers seems to have a bit of an inferiority complex, or something, and is trying to prove his level of experience and intelligence. var f = function() { } and function f() { } are both valid and each have their own unique pros and cons. But to suggest that one syntax is only found in the code of unexperienced developers while the other is the correct way to declare a function is just silliness. — Adam Jenkins
– Adam Jenkins, Commented Nov 29, 2014 at 1:07
Javascript is a rather idiosyncratic language, so it's easy to be an experienced dev but unexperienced at javascript. As far as JS devs are concerned, though, this is not a particularly obscure style. To me, having a strong preference one way or another indicates a degree of discomfort with the language. var f = function () { } is beloved by those uncomfortable with implicit hoisting, and function f() {} is favored by those uncomfortable with thinking of functions as values. After a while, you don't even see the code. All I see is blonde, brunette, redhead... — Chris Martin
– Chris Martin, Commented Nov 29, 2014 at 1:45

Chris Martin · Accepted Answer · 2014-11-29 00:45:25Z

2

observable is a variable. It is declared by a function declaration.

function observable() {
    ...
}

answered Nov 29, 2014 at 0:45

Chris Martin

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4 Comments

Samuel Over a year ago

I always thought that a function declaration could just be called but not used by itself as a variable. I mean, you declare a function and then call it. Or you declared a variable and assign it the function declaration which will create a function declaration and then returns the variable. But in all cases, not simply used a function declaration like a variable. Weird...

Chris Martin Over a year ago

@Samuel Nope, functions are all just values that can be assigned to variables, regardless of how they're declared. The only special caveat about named functions is how they're hoisted (see the link).

Chris Martin Over a year ago

@LightnessRacesinOrbit The attribute of observable named valueHasMutated is assigned by the statement starting with observable.valueHasMutated = ....

Lightness Races in Orbit Over a year ago

@ChrisMartin: Oh, yes, true. This code is kinda weird to read.

Desaroll · Accepted Answer · 2014-11-29 00:47:10Z

0

In Javascript, functions can also be returned. Within the function, he defines the function "observable" which is returned at the end of the function.

Sort to speak, functions are variables too. With a function inside.

answered Nov 29, 2014 at 0:47

Desaroll

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