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Redefining a function in JScript

  • 아티클

Taking a leaf out of Eric's book, I'm going to re-post a question I received in mail here. The question was (essentially):

How do you re-define a function in JScript, and keep the parameters intact?

Simple!

There are (at least) two ways to do it:

1) Just re-define the function using an expression:

// First definition

function
foo(x) { print(x) }

foo(42)
// prints 42

// Second definition

foo =
function
(x) { print(x * 2) }

foo(42)
// prints 84

2) Use Function constructor with more than one parameter:

// First definition

function
foo(x) { print(x) }

foo(42)
// prints 42

// Second definition

foo =
new
Function("x", "print(x * 2)")

foo(42)
// prints 84

Note that you can't do it like this, because the second declaration will clobber the first before your program ever starts to execute:

// First definition (clobbered)

function
foo(x) { print(x) }

foo(42)
// prints 84!

// Second definition (clobbers first)

function foo
(x) { print(x * 2) }

foo(42)
// prints 84

The Function constructor takes the first n-1 parameters and concatenates them with commas, then uses them for the argument list. The nth parameter is the function body. This is kind of cool (in a whacky way) because if you had 3 arguments, you could say:

// Three explicit parameters holding the three

// arguments

foo =
new
Function("x", "y", "z", "print(x + y + z)")

or

// One explicit parameter containing three

// arguments

foo =
new
Function("x, y, z", "print(x + y + z)")

or

// One explicit parameter containing two arguments,

// and a second parameter containing the third argument

foo =
new
Function("x, y", "z", "print(x + y + z)")

You can learn more the Function constructor on MSDN.

Comments

  • Anonymous
    September 13, 2005
    is this ECMA compatible?
  • Anonymous
    September 14, 2005
    Sure