Function that receives a function and a "default parameters" key-value object and returns another function with default arguments set

Question

That sounds kind of complex. The idea here is to write the method "defaultMethod". It receives a function and an object. If the function is a simple add:

function add(a,b) { return a+b;};

When calling

var add_ = defaultMethod(add,{b:9});
add_(10)

The behavior is:

• default value for a: undefined
• default value for b: 9
• received value for a: 10
• received value for b: undefined

and the return must be 19.

The catch is that the method can be called more than once:

var add_ = defaultMethod(add,{b:9}); // set 'b' default value as 9
    add_ = defaultMethod(add_,{b:3, a:2}); // now, set 'b' default value as 3 and 'a' as 2
    let res = add_(10) //sent 'a' value as 10
    expect(res).toBe(13); //10 (received) + 3 (default) 

I wrote it like this:

function defaultMethod(func, params) {
    var funcStr = func.toString();
    let requiredArgs = funcStr
      .slice(funcStr.indexOf('(') + 1, funcStr.indexOf(')')) //get the between parenthesis part
      .match(/([^\s,]+)/g) || []; //resulting in ['a', 'b']

    console.log(requiredArgs)
  
    return function (...args) {
      let calledArgs = args;
  
      if (calledArgs.length < requiredArgs.length) {
        for (let i = calledArgs.length; i < requiredArgs.length; i++) {
          if (calledArgs[i] === undefined) {
            calledArgs[i] = params[requiredArgs[i]];
          }
        }
      }
  
      return func(...calledArgs);
    };
  }
  

It works well for one calling, for example, all of these unit tests passes:

var add_ = defaultMethod(add,{b:9});

it('should return 19', () => {
    expect(add_(10)).toBe(19);
})

it('should return 17', () => {
    expect(add_(10,7)).toBe(17);
})

it('should return nan', () => {
    expect(add_()).toBe(NaN);
})

Although, when we call the defaultMethod one more time, now passing the add_ function, it starts to break. The console.log(requiredArgs) starts to log [...args] instead of ['a', 'b'].

The unit tests are the following:

var add_ = defaultMethod(add,{b:9}); // set b default value as 9
    add_ = defaultMethod(add_,{b:3, a:2}); // now, set b default value as 3 and a as 2

it('should return 13', () => {
    expect(add_(10)).toBe(13); //10 (received) + 3 (default) 
})//this one breaks returning 19

it('should return 5', () => {
    expect(add_()).toBe(5);
})//this one breaks returning NaN

it('should return nan', () => {
    add_ = defaultMethod(add_,{c:3}); // this doesn't do anything because c isn't required
    expect(add_(10)).toBe(NaN);
})//this one breaks returning 19

And I can't figure a way to make it work for more than one calling. Apparently, GPT-4 neither. Any ideas?

edit: I should note that the requirements are:

• to not use global scope (outside of the function)
• we must retrieve the arguments through func.toString()

Answer 1

Do not rely on func.toString(). As you've demonstrated yourself, there are many ways in which this can and does break; it is not a generally solvable problem. Parameter names are an implementation detail and should be considered unreliable.

Instead, have your function accept an object with named properties, you can easily merge those:

function add({a, b}) { return a+b; }
function defaultArgs(fn, def) { return arg => fn({...def, ...arg}); }

With this, your test cases work:

const add9 = defaultArgs(add, {b: 9});
console.log(add9({a: 10})); // 19
console.log(add9({a: 9, b: 6})); // 15
const add32 = defaultArgs(add9, {a: 3, b: 2});
console.log(add32()); // 5
console.log(add32({b: 10})); // 13
console.log(add32({a: 2})); // 4

Answer 2

A solution could be to maintain a registry of functions that are returned by defaultMethod. In your example it would register the function add_ and the parameter names. Then if defaultMethod is called with that function as argument, you can find it in the registry and learn about the parameter names.

So the code would change like this:

function add(a,b) { return a + b; }

const defaultMethod = (function () { // Create a closure for `registry`
    const registry = new WeakMap;
    
    return function (func, params) {
        let requiredArgs = registry.get(func);
        if (!requiredArgs) {
            const funcStr = func.toString();
            requiredArgs = funcStr
              .slice(funcStr.indexOf('(') + 1, funcStr.indexOf(')'))
              .match(/([^\s,]+)/g) || [];
        }

        console.log("parameters are:", ...requiredArgs);
      
        const decoratedFunc = function (...args) {
            let calledArgs = args;
      
            for (let i = calledArgs.length; i < requiredArgs.length; i++) {
                if (calledArgs[i] === undefined) {
                    calledArgs[i] = params[requiredArgs[i]];
                }
            }
      
            return func(...calledArgs);
        };
        // Register the function we are about to return
        registry.set(decoratedFunc, requiredArgs);
        return decoratedFunc;
    };
})();
  
console.log("set defaults for add_ to {b:9}");
let add_ = defaultMethod(add,{b:9});
console.log("call add_(10): expect 19");
console.log(add_(10));
console.log("set defaults for add_ to {b:3, a:2}");
add_ = defaultMethod(add_,{b:3, a:2});
console.log("call add_(10): expect 13");
console.log(add_(10));
console.log("call add_(): expect 5");
console.log(add_());