Question

Ok I’ve been learning some of the more advanced aspects of Javascript and now trying to use this I’m stuck.

Here is my code:

function Data(){}

function init(state){
  var item;
  item=new Data();
  item.fieldrid=17;
  item.description='foo';
  state.push(item);
};


function findInState(state,fieldrid) {
    for (var item in state) {
        alert(item.fieldrid); //prints undefined
        if (item.fieldrid == fieldrid) {
            return item;
        }
    }
    return null;
}

var s=[];
init(s);
alert(s[0].fieldrid); //prints 17 (expected)
alert(findInState(s,17).fieldrid); //exception here. function returns null.

A running example is here at jsbin

Why does this not work? I would expect the alert in findInState to yield 17 but instead it yields undefined.

What am I doing wrong?

Answer 1

for (var item in state)

You should not loop over an array using for..in.
It works as expected when using for (var i = 0; i < state.length; i++).

See https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Statements/for…in:

Although it may be tempting to use this as a way to iterate over an Array, this is a bad idea.