new Function() with variable parameters

Question

I need to create a function with variable number of parameters using new Function() constructor. Something like this:

args = ['a', 'b']; body = 'return(a + b);';  myFunc = new Function(args, body); 

Is it possible to do it without eval()?

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Thank you very much, guys! Actually, a+b was not my primary concern. I'm working on a code which would process and expand templates and I needed to pass unknown (and variable) number of arguments into the function so that they would be introduced as local variables.

For example, if a template contains:

<span> =a </span>  

I need to output the value of parameter a. That is, if user declared expanding function as

var expand = tplCompile('template', a, b, c)  

and then calls

expand(4, 2, 1)  

I need to substitute =a with 4. And yes, I'm well aware than Function is similar to eval() and runs very slow but I don't have any other choice.

Answer 1

You can do this using apply():

args = ['a', 'b', 'return(a + b);']; myFunc = Function.apply(null, args); 

Without the new operator, Function gives exactly the same result. You can use array functions like push(), unshift() or splice() to modify the array before passing it to apply.

You can also just pass a comma-separated string of arguments to Function:

args = 'a, b'; body = 'return(a + b);';  myFunc = new Function(args, body); 

On a side note, are you aware of the arguments object? It allows you to get all the arguments passed into a function using array-style bracket notation:

myFunc = function () {     var total = 0;      for (var i=0; i < arguments.length; i++)         total += arguments[i];      return total; }  myFunc(a, b); 

This would be more efficient than using the Function constructor, and is probably a much more appropriate method of achieving what you need.