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: 
Yes, you **absolutely** should declare the variable. Otherwise you're declaring `v` at global scope which is never good, but it's particularly bad for counting variables which are a single letter long like `v`.

Consider the case where two people get lazy about declaring a variable of the same name:

```
// Your code, which iterates over a nested array

var arr = [ [1, 2], [3, 4] ];

for (i = 0; i < arr.length; ++i) {
  AwesomeLibrary.doSomething(arr[i]);
}

// way down elsewhere, in awesome_library.js...

function doSomething(arr) {
  for (i = 0; i < arr.length; ++i) {
    // Now your `i` is clobbered, and we have a subtle but devastating bug
  }
}

```

This doesn't even require two lazy people: If you work with JavaScript long enough and refuse to declare your variables, you will eventually do this to yourself.
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 !!!