Question: Learning JS here, I run JSLint on this code:

```
/*jslint devel: true*/

function Animal(n) {
    "use strict";
    this.name = n;
    this.prop = "a";
}

var a = new Animal("duppa");

for (v in a) {
    if (a.hasOwnProperty(v)) {
        console.log("property:" + v);
    }
}

```

I get:

```
jslint:test2.js:11:6:'v' was used before it was defined.
jslint:test2.js:11:8:Cannot read property "kind" from undefined

jslint: ignored 0 errors.

```

It obviously complains that I did not declare `v` up front:

```
/*jslint devel: true*/

function Animal(n) {
    "use strict";
    this.name = n;
    this.prop = "a";
}

var a = new Animal("duppa");

var v;
for (v in a) {
    if (a.hasOwnProperty(v)) {
        console.log("property:" + v);
    }
}

```

This shuts JSLint up, but is it really necessary? In general I try to follow good conventions but is this one really necessary in JS? E.g. Python lives happily without such stuff (`for x in range(10)...` etc).

Answer: 
There are 10 types of people in the world. Those who understand why you declare variables in javascript and those who have regular sex. (**Just smile**)

You must understand that every function have their own scope and you must use this scope. If you don't use declaration inside your function you change the global state, and it affects of course on many things.

So use **var** and don't create global variables !!!
It is always a better practice to define your variables before using them. Here javascript's for loop requires you to define i, because it is in the for loop, use 

```
var v;
for( v in a)

```