Missing values

The most common way a correct-looking rule does nothing, forever, with no error.

A stat is 0. A field is null.

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

null loses every comparison

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

So someone without an infection field does not match infection < 5.

Usually that's right: it stops a cure rule firing on every healthy person in the world. But if you meant "everyone below 5, including people who've never been infected", your rule matches nobody.

Why you get no error

• A missing path reads as undefined.
• A condition treats undefined as "closed".
• An expression that throws gives nothing, and the engine uses its default.

Three separate mechanisms, all silent. Editor completion only offers paths that exist, so if you typed a path by hand and completion didn't offer it, that's probably your bug.

Fixes

Count "doesn't have it" as passing:

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

Treat missing as zero:

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

Check the plugin that owns it is installed:

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

Why missing isn't just 0

Because 0 means something. Satiety 0 is starving. Satiety null is this game has no hunger. If missing read as zero, every character in a game without hunger would be starving.