CODEWITHSUNDEEP
javascriptclosures
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Reputation: 14271

Is it possible to Detect Closure Programmatically?

I have a function which allows user to pass a function. 

function withPredicate(func){
    //...
}

Inside my function, I need to detect whether the func that user pass in has closure or not.

It is not enough to get the function name and search it in window scope. User might pass in an anonymous function without closure like:

var func = function(x){return x;};
withPredicate(func);

EDIT:

I think I need to implement a function which takes a function as a argument and return bool. 

function hasClosure(func){
    //...
}

so several test cases are:

hasClosure(function(x){return x;}); //return false

var something = 30;
var func1 = function(x){return x.age < something;} 
hasClosure(func1);   //return false

var Closure = function(){
    var something = 18;
    var itself = function(x){
        return x.age < something;
    };
    return itself;
};

var func2 = new Closure();
hasClosure(func2); //return true

The last one return true because func2 is not top-level function. When I see some free variable inside the function body like something, it may resolve to the variable defined in its closure other than the one defined in window.

Actually, what I need to do now is to do some manipulations based on the func that has been passed to my function. I can use JSLint to get the undeclared variable. But I also need to resolve these variables. It is acceptable that I can only resolve variables in global scope. But I still need a way to make sure that resolving these variables in global scope is correct. So I need to detect closures.

Is it possible to do that programmatically in javascript?

Upvotes: 2

Views: 951

Answers (2)

Alex Netkachov
Alex Netkachov

Reputation: 13532

The correct answer highly depends on what a "closure" means in this context. The function from the question does not use the variables from the outer scope so, roughly, closure is not created for this function. But strictly speaking, closure is created for any function and binds definition context with the function. So whatever you want to detect, it requires a detailed specification.

But I can assume that the closure here means whether the function references variables declared in outer scope (just like https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Closures specifies). One of the static for analysis libraries can be used for this. For example, JSLint reports used but undeclared variables. Assuming that undeclared variables are just from the upper scope, this indicates that closure is created.

Upvotes: 0

megawac
megawac

Reputation: 11353

Alright this is really hacky and probably not worth the effort in actually writing unless your linting or something :)

Basically what you're going to have to do to determine if a function is a closure is call func.toString() which will give you the source of the function. You can then parse the function using some sort of parser which determines all the variables of the function func. You also need to determine and track how these variables are defined. In your criteria in op the criteria for it being a closure is having a variable that is defined outside of function scope and outside of window scope (thus closure scope). So if you can find any variables defined outside of these two scopes we've found a closure.

So heres the pseudocode of hasClosure(), have fun implementing. 

func hasClosure(func)
   source = func.toString(); //eg "function x(a) {return new Array(a)}"
   variables = parseJSForVariables(source)//finds a and Array
   for variable in variables:
      if(variable not in window)
          if(variable not defined in function)
              return true //we got a closure
  return false //not a closure

Upvotes: 1

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