Function Declaration in JavaScript

It is easy to declare a function in JavaScript. And everyone knows it:

Standard or Procedural Function

; function func()
{
...
}

The problem of it is this function cannot be passed as parameter in another function. To be able to do that, you will need to varibalize it:

Varibalized Function, or Function Literal, Function Reference, Function Pointer

; var func = function()
{
...
}

Everyone does that too. Wait a moment. This form for declaration indeed is just set to fail if you use it carelessly. You may get following error massage when sequence to execute programming block becoming sensitive, such as work with setTimeout() and setInvertal() or inside of a function:

[This function] is not defined.

If you include this form of function with setInterval or setTimeout, you may get error message:

useless setInterval call (missing quotes around argument?)

This is because you will need to put it in right sequence to make sure the memory has load the function first.

So, it is suggested in general you should use orthodox function declaration method. If you use variablized function declaration method, please make sure it has been put into right place in the sequence of programming blocks.

Have a look at following case:

; var func = func()

; function func()
{
...
}

No, this is not a function declaration method for first statement. It just get the return value from the function func().

Apart from above mentioned popular method, here is

Function Object

; var func = new Function()
{
...
}

It is not recommended to use because it would take longer time to process, unless for special purpose, such as reconstruct to function from variablized function and variablized parameters. For instance:

; var reconstructedVar = new Function([para1, para2,...paran], functionBody)
{
...
}

Please take note, all parameters and function body must be in string or reference. The other way to reconstruct the function is to do it by yourself. For details, please see Passing Function in JavaScript.


http://www.permadi.com/tutorial/jsFunc/index.html
http://www.hunlock.com/blogs/Functional_Javascript
http://www.dustindiaz.com/javascript-function-declaration-ambiguity/
http://osdir.com/ml/jQuery/2009-11/msg01942.html
Javascript Error Message: is not a function

Passing Function in JavaScript

To pass function like a reference, one needs first to make the function a variable reference:

; var funcationName = function(argu) { ... }

Then, it can be passed:

; function anotherFunc(passingFunc)
{
; passingFunc(para)
}

Use it just like the function itself. Indeed, if there is no parameter for functionName(), you will still need to use it as function:

; function anotherFunc(passingFunc)
{
; passingFunc()
}

Function passing even can be used to across the html pages, just like variable reference passing:

; function anotherFunc(passingFunc)
{
; parent.passingFunc()
}

In HTML, following syntax is used to invoke the function:

onkeypress="anotherFunc(funcationName)"

It is very important, the sign of () could NOT be put in invokin statement in HTML, something like:

onkeypress="anotherFunc(funcationName())"

Now, this syntax does create confusion when you want to pass the parameter as will, for instance, you want to call anotherFunc(funcationName(argu)). The ways to overcome it is:

1. For static passing function with dynamic passing varialbe: Declare argu as global variable. Codes are as follows:

; var para

; var funcationName = function(argu) { ... }

; function anotherFunc(passingFunc)
{
; passingFunc(para)
}

onkeypress="para=something;anotherFunc(funcationName)"

3. If you want a complete dynamic passing function together with a dynamic passing variable, you will need to create third function:

; var para

; var funcationName = function(argu) { ... }

; var thirdFunc = function()
{
...
; funcationName(para)
...
}

; function anotherFunc(passingFunc)
{
; passingFunc()
}

onkeypress="para=something;anotherFunc(thirdFunc)"

Function thirdFunc() indeed is where you should play with.

Another approach is to write a function to reconstruct the function from variables:

; function reconstructFunc(func, totalPara, para1, para2, para3)
{
; var newFunc
; if (0==totalPara) newFunc = func
else if (1==totalPara)
{
; newFunc = function()
{
; func(para1)
}
}
else if (2==totalPara)
{
; newFunc = function()
{
; func(para1, para2)
}
}
else
{
; newFunc = function()
{
; func(para1, para2, para3)
}
}
; return newFunc
}

It is very important when passing function, the function must be properly declared. Please refer to http://koncordpartners.blogspot.com/2010/04/function-declaration-in-javascript.html. This article also covers another method to reconstruct the function from variablized function and parameters.

If the function reference/literal had passed across HTML pages, in IE 8 it might general error as "Function expected". To overcome this, you need to set up a local variablized function to call access remote function.