variable declaration in javascript

Question

I have been doing something like this for a while and I have never seen any errors. But, unfortunately, I have never been able to explain why this works.

The first line creates a variable which points to a function. The second line just adds func2 to func1 separated by a dot and defines a function.

If I add a var in front of func1.func2 then I see a compilation error.

func1.func2 = function(){};

Error

SyntaxError: missing ; before statement

var func1.func2 = function(){};

What type was func1 on first line 1 and what did it become on line 2.

var func1 = function(){};
func1.func2 = function(){};

func1 is a Function Object and as such, can have properties dynamically assigned — Phil
– Phil, Commented Nov 11, 2013 at 22:30
When you do var func1, you are creating a new variable called func1. func1.func2 doesn't create a new variable, it adds a property to a variable that already exists. — gen_Eric
– gen_Eric, Commented Nov 11, 2013 at 22:31
If you add a var in front of it, then you're creating a variable and variable names can't have periods. That's why you get an error. — Vic
– Vic, Commented Nov 11, 2013 at 22:33

elclanrs · Accepted Answer · 2013-11-11 22:31:57Z

1

A function in JavaScript is an object too, that can have properties:

function func1(){} // a function object
console.log(func1.name); //=> "func1"

An anonymous function doesn't have a name but it's still a function object so:

What type was func1 on first line 1 and what did it become on line 2.

Still a function, with a property func2 containing another anonymous function.

answered Nov 11, 2013 at 22:31

elclanrs

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Comments

Evan Davis · Accepted Answer · 2013-11-11 22:30:32Z

1

func1 is a function on both lines. You can confirm this using typeof:

var func1 = function(){};
typeof func1; // "function"
func1.func2 = function(){};
typeof func1; // "function"

answered Nov 11, 2013 at 22:30

Evan Davis

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Comments

RobG · Accepted Answer · 2013-11-11 22:44:34Z

1

One form of variable statement is:

var identifier [optional assignment];

The identifier in a variable statement must meet the rules for identifier names, one of which is that they can't contain a "." character. So:

var func1 [...];

is OK but:

var func1.func2 [...];

is not. The interpreter stops at the identifier because it's a syntax error and that's it, it can't proceed with the assignment.

answered Nov 11, 2013 at 22:44

RobG

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Comments

Giuseppe Pes · Accepted Answer · 2013-11-11 22:44:49Z

1

func1.func2 = function(){};

If func1 is not defined you are trying to access a property of undefined which causes an exception to be thrown.

var func1.func2 = function(){};
// SyntaxError: Unexpected token .

JavaScript syntax does not allow a dot to be inserted in a variable's name and therefore the interpreter returns a syntax error error.

var func1 = function(){};
func1.func2 = function(){};

A function in javaScript is a Function object, therefore, as all javascript objecta a method can be added dynamically to it at run-time. So func1 is Function Object and func2 is another function object. This is the correct way to achieve what you are trying to do.

answered Nov 11, 2013 at 22:44

Giuseppe Pes

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1 Comment

Felix Kling Over a year ago

Actually, if func1 is not defined then you'd get a reference error. The "trying to access X of undefined" error usually happens when you are trying to access a non-existing property of an object. Because in that case, the return value is actually undefined.

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