# Missing values

This page exists because it is the single most common way a correct-looking rule does
nothing at all, forever, with no error anywhere.

## A stat is `0`. A field is `null`.

```js
$.stat('nonsense')   // 0
$.field('nonsense')       // null
```

That difference is the whole page.

## `null` loses every comparison

Including the ones you would expect it to win:

```js
null < 5    // false
null > 5    // false
null == 0   // false
```

So a character with no `infection` field **does not satisfy** `infection < 5`.

That is usually what you want — it is what stops a cure-the-infected rule from firing on
every healthy person in the world. But if you *wrote* that rule expecting it to catch
everyone, it fires on nobody and looks broken.

## The symptom

A rule that never fires, no error, nothing in any log. Because:

- reading a missing path gives `undefined`
- `conditionSatisfied` treats `undefined` as "the gate does not open"
- an expression that throws evaluates to nothing, and the caller uses its default

Three separate mechanisms, all of which turn a mistake into silence. Editor completion
only offers paths that exist, which is your first defence — if you typed a path by hand
and it is not in the completion list, that is the bug.

## Getting it right

**Ask whether the field exists**, when "nobody has this yet" should count:

```js
!$.hasField('infection') || $.field('infection') < 5
```

**Give it a floor**, when you want a missing field to read as zero:

```js
($.field('infection') || 0) + 1
```

**Check the plugin is installed**, when the field belongs to someone else:

```js
$.hasPlugin('hunger') && $.field('satiety') < 20
```

## Why not just make missing fields `0`?

Because `0` is a real value with real meaning. A satiety of `0` means *starving*; a satiety
of `null` means *this game has no hunger system*. Collapsing those would make every
character in a game without hunger permanently starving, which is exactly the bug that
`$.hasPlugin()` exists to prevent.

## Next

[Conditions](/docs/conditions) — where these checks live.