JavaScript Function arguments

Question

I am trying to create a function that deserialises a JSON object and creates some functions, but I want to be able to specify a variable set of arguments.

For example, in JSON I want to specify something like:

{
  "handler" :
  {
    "args" : ["evt","value"],
    "content" : "console.log(value);"
  }
}

And when I parse the object, I will do something like:

var myFunction = new Function(handler.args,handler.content);

But the challenge is that each argument is supposed to be a n strings followed by the content of the function as the last argument. Is there any easy way of specifying n number of arguments in a new Function()?

Felix Kling · Accepted Answer · 2011-06-27 21:16:28Z

5

To solve the technically issue: You can use apply ^[docs].

handler.args.push(handler.content);
var myFunction = Function.apply(null, handler.args);

However the question is why you are doing something like this? What is the context? Spontaneously I would say you should consider another solution for whatever problem you are trying to solve ;)

edited Jun 27, 2011 at 21:16

answered Jun 27, 2011 at 21:14

Felix Kling

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3 Comments

Ruan Mendes Over a year ago

This calls the function immediately, OP is asking how to use the function constructor to generate a function

Felix Kling Over a year ago

@JuanMendes: ? This calls the function constructor. Function(a, b, c) is the same as Function.apply(null, [a, b, c]).

Ruan Mendes Over a year ago

sorry, I did misread the code. I'm still trying to figure out what OP means by the question

MooGoo · Accepted Answer · 2011-06-27 22:33:31Z

1

According to MDN

Parameters

arg1, arg2, ... argN

Names to be used by the function as formal argument names. Each must be a string that corresponds to a valid JavaScript identifier or a list of such strings separated with a comma; for example "x", "theValue", or "a,b".

So the arguments list can either be one or more strings seperated by commas, or just one string with each identifier in it seperated by commas.

Also since

['evt', 'value'].toString() == 'evt,value'

Simply passing your handler.args array as the first argument to the new Function constructor should work exactly as you want it to

new Function(handler.args, handler.content);

Internally, new Function casts every argument to a string if it is not already one. So conceivably something like this would also work

new Function({ toString: function() { return 'a,b,c' } }, 'return a+b+c');

Not that I'm suggesting you do anything silly like that.

This works in every browser I've tried including IE

edited Jun 27, 2011 at 22:33

answered Jun 27, 2011 at 22:27

MooGoo

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3 Comments

Felix Kling Over a year ago

I don't think this will work. new Function('evt, value', handler.content); is different than new Function('evt', 'value' , handler.content); I don't think Function calls split(',') on its arguments. And evt, value by itself is an invalid parameter name (will, it will be interpreted as two parameters). But you cannot conclude that it having a string in this format has the same effect.

MooGoo Over a year ago

@Felix Kling - It most certainly does work, because the MDN documentation said it would. Regardless of how it works internally, the docs clearly say 'a','b' is interpreted the same way as 'a,b'. I can conclude exactly what I said, because I've tried it and it works. Why don't you try it yourself?

Kitson Over a year ago

Actually, I was going for Felix's answer, but in this particular instance, this works out cleaner for me in my code, so I am going to accept this one. Thanks for the answer!

ChaosPandion · Accepted Answer · 2011-06-27 21:15:43Z

1

I think the simplest route would be to combine the 2 properties. Then use apply to construct the function.

var x = {
  "handler" :
  {
    "constructorArgs" : [
        "evt",
        "value",
        "alert(value);"
    ]
  }
};

var f = Function.apply(undefined, x.handler.constructorArgs);
f(1, 2);

To keep it similar you can use Array.prototype.concat.

var x = {
  "handler" :
  {
      args: [ "evt", "value" ],
      content : "alert(value);"
  }
};

var f = Function.apply(undefined, x.handler.args.concat(x.handler.content));
f(1, 2);

answered Jun 27, 2011 at 21:15

ChaosPandion

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Comments

Ruan Mendes · Accepted Answer · 2011-06-27 21:17:40Z

0

Can;t you just make the body of your functions work with the arguments property?

http://jsfiddle.net/9XcEb/

var add = new Function(
    "var total=0;"+
    "for (var i=0; i < arguments.length; i++) {"+
       "total+=arguments[i]"+
    "};"+
    " return total"
);

alert(add(3,4,5,6));

answered Jun 27, 2011 at 21:17

Ruan Mendes

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