Typeof for javascript function

Question

When I declare a function like this:

function x() { return 123 };

Then:

typeof x; //return "function"
typeof x(); //return "number"

That's ok, but once I create a variable like this:

var y = function x() { return 123 };

It becomes:

typeof y; //return "function"
typeof y(); //return "number"
typeof x; //return "undefined"
typeof x(); //return error

Why does x lose his function? Please helpp

I am getting different results..Tried in chrome and mozilla.. — Rayon
– Rayon, Commented May 17, 2015 at 8:16
@RayonDabre I don't, I tried both too. What exactly did you try? — Mörre
– Mörre, Commented May 17, 2015 at 8:18
See kangax.github.io/nfe --- under the headline "Named function expressions" this exact thing is mentioned (end of that section): "An important detail to remember is that this name is only available in the scope of a newly-defined function; specs mandate that an identifier should not be available to an enclosing scope:" — Mörre
– Mörre, Commented May 17, 2015 at 8:19
@RayonDabre That is completely different. What are you trying to show? Of course your "x()" is defined -- you do so at the top! See my comment and the link above. — Mörre
– Mörre, Commented May 17, 2015 at 8:23

Quentin · Accepted Answer · 2015-05-17 08:27:17Z

1

Why does x lose his function?

A function declaration creates a variable of the same name in the current scope.

A named function expression does not (except in certain older versions of Internet Explorer, which is a bug).

That's just how function expressions are supposed to work.

answered May 17, 2015 at 8:27

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Quentin

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6 Comments

bitifet Over a year ago

I usually say that «function x(){}» is only a shorthand for «var x = function x(){}». The first 'x' is the variable name, which is the name of the variable which holds the function in it's declaration time. The second 'x' is the actual name of the function, which is what you will see in error traces and debugging data despite the variable or method used to call it. …also, function name can be used to call itself (recursively) inside the function whithout caring for if the initial holding variable is deleted or reused.

Quentin Over a year ago

I usually say that «function x(){}» is only a shorthand for «var x = function x(){}». — Well, it isn't. Aside from all the other differences you've already listed, function declarations are hoisted and function expressions are not.

piuspbd Over a year ago

@bitifet does it mean that whatever name I use to declare function, it will lose its function if I declare it as a variable in another name? And the function itself will belong to the new variable

bitifet Over a year ago

True. I tryed to explain clearly why there is the two forms and did wrong. It is actually not "only" that...

Quentin Over a year ago

@Pius — No. Assigning a function to a variable won't remove all other references to it. It's just that creating a function using a function expression won't create a variable of the same name in the current scope.

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