Cannot read property of array

Question

In my app.js, I have the following:

post.find({'global': true}).sort('date').exec(function(err, data) {
  for (var i = 0; i <= data.length; i++) {
    console.log(data[i].email);
    //socket.emit('Post', {name: data[i].name, cont: data[i].cont, date: data[i].date});
  }
});

When I use

console.log(data[i]);

I get my data in the form of

{name: blah, cont: blah, email: blah, etc}

But when i try emitting the data

data[i].attribute

I get the error

TypeError: Cannot read property 'attribute' of undefined

I know the data is there, as I am able to log it in the console. Why can't I access the specific attribute of the array? Any ideas?

hexacyanide · Accepted Answer · 2013-10-04 03:02:13Z

1

It might be because of how you're using your loop. Your logic with <= will causes the loop to run one extra time, therefore accessing an array value that doesn't exist. This is a case of what's happening:

var data = [{ foo: 'bar1' }, { foo: 'bar2' }];
for (var i = 0; i <= data.length; i++) {
  console.log(data[i].foo);
}

The loop will run three times instead of twice, causing this to happen:

console.log(data[0].foo); // bar1
console.log(data[1].foo); // bar2
console.log(data[2].foo); // TypeError: Cannot read property 'foo' of undefined

To fix this, change your loop to for (var i = 0; i < data.length; i++).

Also, a debugging tip: you should try using output when inspecting errors like yours, even if data was an array with a length of a thousand, the error would still only be thrown on the last iteration, and with socket.emit() it would appear as if the loop only ran once.

edited Oct 4, 2013 at 3:02

answered Oct 4, 2013 at 2:57

hexacyanide

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