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 !!!
Remember that JSLint doesn't complain because of some obscure academic reason but because it protects you from major problems your application may potentially suffer from.

You declare a variable to contain the variable in the current scope. It's a safeguard for you and your program as well making it more readable code (global vars appearing out of nowhere are always confusing).

Imagine if you had a global variable v in your application and then used the same nomenclature (`v`) for iteration in a function. Javascript will automatically assume that the global variable is being used and you'll find yourself with unwanted values in your global. Having said that, the less stuff you put in the global namespace the better.