Inner function taking arguments from outer

Question

I don't get how the inner function gets passed the arguments from .sort() method. I know that .sort() passes the values to createComparisonFunction() , but how do they end up in the inner function? Does it just take any unused arguments from the outer function?

I'd like to understand that behavior.

    function createComparisonFunction(propertyName) {

        return function(object1, object2){
            var value1 = object1[propertyName];
            var value2 = object2[propertyName];

            if (value1 < value2){
                return -1;
            } else if (value1 > value2){
                return 1;
            } else {
                return 0;
            }
        };
    }

    var data = [{name: "Zachary", age: 28}, {name: "Nicholas", age: 29}];

    data.sort(createComparisonFunction("name"));
    alert(data[0].name);  //Nicholas

    data.sort(createComparisonFunction("age"));
    alert(data[0].name);  //Zachary     

Answer 1

No, the .sort() function does not pass parameters to "createComparisonFunction". Instead, "createComparisonFunction" does exactly what its name suggests: it creates a function and returns it. The returned function is the one called repeatedly by the .sort() method.

Note that in the call to .sort() :

data.sort( createComparisonFunction("name") );

the "createComparisonFunction" is being called . That's what the parenthesized argument list (with the single parameter "name" ) means — call this function . That happens before the runtime invokes the .sort() method. What's passed to .sort() is the return value, which is itself a function.

The most interesting thing going on is that the returned function — which takes two parameters, as a sort comparator should — has access to the parameter originally passed to "createComparisonFunction". That's because a function that's returned from another function retains access to its original creation-time local variable context.

Answer 2

You can use methods call and apply. For examplem, createComparisonFunction.apply(someNewContext, someArguments). First argument for both methods should be context.