Javascript: return function with predefined arguments

Question

I have a function like

function a (p1, p2) { /* ... */ }

and in some scope want to get something like this:

function b (/* no params! */) { return a (my1, my2) }

where my1 and my2 are defined somehow in this scope. So I should get a parameterless function b, which when called calls a with fixed parameters my1 and my2. Now, the question is, why this is not right, and which is :)

UPD: Ok, I had some callbacks in those params, now found out, how to process them all. What I missed was to apply the technique twice. Thank you.

JW. · Accepted Answer · 2009-03-10 16:54:57Z

25

Just make a function that returns a new function b:

function getB(my1, my2) {
  return function() {
    a(my1, my2);
  }
}

and use it like this:

var b = getB(my1, my2);

answered Mar 10, 2009 at 16:54

JW.

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Comments

Matthew Marshall · Accepted Answer · 2009-03-10 16:54:34Z

3

Write a function that takes my1 and my2 and creates function b:

function make_b(my1, my2){
   return function b(){ return a(my1, my2);};
}

answered Mar 10, 2009 at 16:54

Matthew Marshall

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Comments

Chetan S · Accepted Answer · 2009-03-10 17:05:15Z

2

I'm not sure if I understood you correctly, but you can look at a concept called currying. It is possible to curry a function in Javascript such that you can obtain another function with some or all of the parameters preset.

You can look at an implementation here.

If you are using Prototype, you can obtain the function b this way.

var b = a.curry(arg1, arg2);

now calling b() is the same as calling a(arg1, arg2)

edited Mar 10, 2009 at 17:05

answered Mar 10, 2009 at 16:57

Chetan S

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Comments

JoshBerke · Accepted Answer · 2009-03-10 17:03:43Z

1

Why is this not right? Are you getting an error? I tried to reproduce a working sample based one your description and what I have works so perhaps you could post a more detailed sample:

<script>

function a(p1, p2)
{
 return 'hello ' + p1 + ' and ' + p2;
}
function b(my1,my2)
{
 return function()
        {
          return a(my1,my2);
        }
}

var wrapper=b();
alert(wrapper());
</script>

Edit

Based on your comment to another answer I updated my sample to show how you can wrap a function.

Edit 2

Ok so I replaced them with variables;-)

edited Mar 10, 2009 at 17:03

answered Mar 10, 2009 at 16:52

JoshBerke

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1 Comment

stanch Over a year ago

the problem is - my1 and my2 are not constants as in your example 'josh' and 'fred'. The must be evaluated at the time of creation of b. e.g. var v='some text'; function b () {return a(v);}

Diodeus - James MacFarlane · Accepted Answer · 2009-03-10 16:52:52Z

0

Put the values in an array (or JSON) and return the array.

function a (p1,p2) { 
   var my = []
   my[0] = p1
   my[1] = p2
   return my
}

function b () {
    return a(my1,nmy2)
}

Maybe I'm missing the point in your example, I don't know.

edited Mar 10, 2009 at 16:52

answered Mar 10, 2009 at 16:47

Diodeus - James MacFarlane

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1 Comment

stanch Over a year ago

It seems to me that you got me wrong (or maybe I was not strict enough). What I want is - having a function1, that takes some parameters, to generate a function2, that takes no parameters and returns function1 with some fixed parameters, not constant, certainly, they must be evaluated at that moment

Michael Brewer-Davis · Accepted Answer · 2009-03-10 17:02:27Z

0

There appear to be three parts:

Defining a:

  function a(M, N) {
    return M + N;
  }

Defining b:

  var X = 10;
  var Y = 25;
  var b = function() {
   return a(X, Y);
}

Using b:

  var thirtyFive = b(); // thirtyFive = 35

answered Mar 10, 2009 at 17:02

Michael Brewer-Davis

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Comments

user784509 · Accepted Answer · 2011-06-07 05:04:51Z

This example uses the assignment of an onclick event handler during page load to demonstrate how to create a function reference containing embedded references to then local values in the setup function.

<html>
<head>
<title>Scope passing Test</title>
<script type="text/javascript">
// Your target function
function a(p1,p2)   {   alert("p1="+p1+ " p2="+p2); }


function yourSetupFunction()
    {
// Declare some local variables that your "a()" function cannot see to test.
var my1 ="Look Ma"  ;
var my2 ="No hands" ;

//  Snag a reference to the <a id="testA"></a> element  
var oAelement=document.getElementById("testA");

oAelement.addEventListener( "click"     ,
                (function(scopePass_My1,scopePass_My2)
                        {
                        return  function()
                            {
                            a(scopePass_My1,scopePass_My2)
                            }
                })(my1,my2) ,true);
    }
</script>
</head>
<body onload="yourSetupFunction()">
<a id="testA" href="#" onclick="return false;">Click to Test</a>
</body>
</html>

The actual magic is just a few lines

(function(scopePass_My1,scopePass_My2)
            {
            return  function()
            {
                a(scopePass_My1,scopePass_My2)
            }
    })(my1,my2)

The first set of parens cause the contents to be evaluated to a function reference.

The second set of parens ... (my1,my2) ... cause the function reference to be called which returns another function reference which can see the parameters passed in ... scopePass_My1,scopePass_My2 ... and is able to pass these to your target function a(p1,p2)

Hope that helps.

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