Javascript array.length property

Question

I'm working through the fabously-named learnyounode. #9 is "JUGGLING ASYNC" and the instructions are to take in 3 urls as params and output the content in parameter order. I got it working by including a counter, but my original did not work:

var http = require('http'),
  bl = require('bl'),
  store = [],
  count = 0; //added later

process.argv.forEach(function(element, index, array) {
  if (index > 1) {
    http.get(element, function(response) {
      response.pipe(bl(function(err, data) {
        store[index] = data.toString();
        count++; //added later
        if (store.length == 3) {
            store.forEach(function(e,is,a) {
                console.log(e);
            });
        }
      }));
    });
  }
});

Now the thing works fine if you replace store.length with count on line 12, I just can't figure out why .length on the array wasn't enough. Anyone know?

I acknowledge a for loop instead of process.argv.forEach would have saved 2 loops. — Matt Murphy
– Matt Murphy, Commented Dec 17, 2014 at 21:28
I cannot tell what your question is. What was your code originally, what did you expect, and what did you get instead? — Phrogz
– Phrogz, Commented Dec 17, 2014 at 21:29
store.length can give odd responses; index should change consistently in the forEach loop. Try index ... — bob-fornal
– bob-fornal, Commented Dec 17, 2014 at 21:31

redbmk · Accepted Answer · 2014-12-17 21:33:25Z

4

In javascript arrays, when you give an empty array an item at index 5, it implies that the array is 6 items long, with items 0-4 being undefined.

For example

var x = [];
console.log(x.length); //prints 0;
x[2] = "hello, world";
console.log(x.length); //prints 3;

You're skipping the first two elements but still using index, so when you enter your first element into store, you're entering it into store[2], making the length 3, when count is only 1.

answered Dec 17, 2014 at 21:33

redbmk

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4 Comments

Matt Murphy Over a year ago

Ah, this was a surprise to me... (but perhaps it shouldn't) var x=[]; x["a"]=1; console.log(x.length); // prints 0

redbmk Over a year ago

Yeah javascript has a pretty loose interpretation of objects. I just used something in a loop the other day like x[otherArray.indexOf(item)] = item. If it can't find it in the other array it goes into -1, which doesn't count towards length so essentially gets ignored.

Matt Murphy Over a year ago

Playing around with it further, I started over with:

var x=[]; x['a']='a'; console.log(x); // yields [ a: 'a' ] console.log(x.length) // again, 0 x[1]=1; console.log(x); // yields [ , 1, a: 'a' ] console.log(x.length); // yields 2 for the empty 0 place and the 1 place

...anyway, I though that was interesting. It's almost like these alpha indexed array items are actually object items, and thus don't get counted. That's probably a silly way of thinking of it...

redbmk Over a year ago

The Mozilla Developer Network (MDN) has some really great documentation on javascript and HTML. It might help to read up on Arrays (there's also a specific section for the relationship between length and numerical properties). A [] is actually a simple object that is an instance of the Array class, with many functions and properties.

Seva Arkhangelskiy · Accepted Answer · 2014-12-17 21:49:41Z

0

You should use your count variable, because:

When you you do store[1] = data.toString();, for example, you get an array [undefined, 1] with length = 2. And you want it to be 1.

edited Dec 17, 2014 at 21:49

answered Dec 17, 2014 at 21:34

Seva Arkhangelskiy

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5 Comments

Bergi Over a year ago

No. The instructions are clear: output the results in parameter order.

Bergi Over a year ago

If you push it to the end of the array, you'll have an indeterministic order.

Seva Arkhangelskiy Over a year ago

@Bergi, it's an array, not an object. Arrays always keep the order. Regular objects - don't.

Seva Arkhangelskiy Over a year ago

@Bergi, or could you please give me a link where I can read about it, if I'm mistaken.

Bergi Over a year ago

I'm not saying the array is forgetting the order, I say that you are appending them in the possibly wrong order. Notice that http.get and bl are asynchronous.

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