How does Javascript interpret this parameter error?

Question

function each(collection, callback) {
  if (Array.isArray(collection)) {
    for (var i = 0; i < collection.length; i++) {
      callback(collection[i]);
    }
  }
  else {
    for (var prop in collection) {
      callback(collection[prop], prop, collection);
    }
  }
}


var array = [1, 2, 3, 4, 5];


function reduce(collection, callback, initial) {
  var current = initial;
  each(collection, function(current, e) {
    current = callback(current, e);
  })
  return current;
}

console.log(reduce(array, function(a, b) { return a + b }, 0));  -->>> 0

I'm trying to rewrite the underscore each/reduce functions, and use the each function in reduce. I know that I have a mistake in there-- (current should not be in the each callback function) it should be just be

each(collection, function(e) {
  current = callback(current, e);
})

and that returns 15 as it should, but why does it return 0 when you do add that current in there as a parameter? Shouldn't it just return NaN? As the last part of the loop will try to add 5 and undefined which is NaN.

Just a comment, you really should check .hasOwnProperty() in your each() function for the case when the collection is not an array — slebetman
– slebetman, Commented May 15, 2015 at 4:38
Because when you add current as a parameter, you override the outer scope variable, and end up never changing it's initial value — Patrick Evans
– Patrick Evans, Commented May 15, 2015 at 4:43
Not related to your problem, but if you are going to pass prop and collection to the callback in the case of an object, shouldn't you also pass i and collection in the case of an array? — user663031
– user663031, Commented May 15, 2015 at 4:51

Bergi · Accepted Answer · 2015-05-15 04:50:04Z

3

The thing is that as soon as you add current to the parameter list of your callback function, you do have two variables - with the same name, but in different scopes. The one in the callback does shadow the one in the reduce function.

So your callback reducer is called with the element that each passed to your callback and undefined, but when assigning the result (NaN) to current it will just assign to the local variable of your each callback.

The outer current variable will stay completely unaffected, and when it is returned from reduce it still holds the initial value it was initialised with - 0.

answered May 15, 2015 at 4:50

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How does Javascript interpret this parameter error?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related