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# JsonLogic Overview

> A short tour of the standard JsonLogic expression language used to author Semantius business rules.

- **URL**: https://www.semantius.com/docs/business-logic/jsonlogic

---

[JsonLogic](https://jsonlogic.com/) is a small, portable rule language that expresses boolean tests, arithmetic, and data lookups as plain JSON. Semantius uses it as the substrate for all four [business-logic extension points](https://www.semantius.com/docs/business-logic.md), with a few [Semantius-specific additions](https://www.semantius.com/docs/business-logic/extensions.md) layered on top.

This page is a quick orientation. For the full operator catalog, refer to the upstream documentation:

- Project home: [jsonlogic.com](https://jsonlogic.com/)
- Operator reference: [jsonlogic.com/operations.html](https://jsonlogic.com/operations.html)

## The shape of a rule

A JsonLogic expression is a single JSON object whose top-level key is the operator name and whose value is the operator's argument list. Operators nest freely:

```json
{ "==": [{ "var": "status" }, "active"] }
```

This reads as: take the value of the `status` field of the current row, compare it to the string `"active"`, return `true` if they match.

Anything that is not an object with one of the known operator keys is returned as-is. That includes literals (`42`, `"hello"`, `true`, `null`) and plain arrays.

## Common operators

| Operator                                         | Purpose                                                                                                                                              |
| ------------------------------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------- |
| `var`                                            | Read a field from the current context. Dotted paths walk into nested objects: `{"var": "customer.email"}`.                                           |
| `==`, `!=`, `<`, `<=`, `>`, `>=`                 | Equality and ordered comparison.                                                                                                                     |
| `and`, `or`, `!`                                 | Boolean composition and negation.                                                                                                                    |
| `if`                                             | Three-argument ternary: `{"if": [<cond>, <then>, <else>]}`. Accepts more arguments for `if / elseif / else` chains.                                  |
| `+`, `-`, `*`, `/`, `%`                          | Arithmetic.                                                                                                                                          |
| `cat`                                            | String concatenation (treats `null` as `"null"`, unlike the Semantius [`concat`](https://www.semantius.com/docs/business-logic/extensions.md) extension). |
| `in`                                             | Membership test in an array or substring test in a string.                                                                                           |
| `map`, `filter`, `reduce`, `all`, `none`, `some` | Array combinators.                                                                                                                                   |
| `merge`                                          | Flatten arrays.                                                                                                                                      |
| `missing`, `missing_some`                        | Report missing required fields.                                                                                                                      |

## Samples

**Reject negative quantities.**

```json
{ ">=": [{ "var": "quantity" }, 0] }
```

**Status defaults to "draft" when blank.**

```json
{ "if": [
  { "or": [{ "==": [{ "var": "status" }, null] }, { "==": [{ "var": "status" }, ""] }] },
  "draft",
  { "var": "status" }
] }
```

**Order must have at least one line item.** Using the standard `missing` operator to report a required field:

```json
{ "==": [{ "missing": ["customer_id"] }, []] }
```

**Compute total from subtotal and tax.**

```json
{ "+": [{ "var": "subtotal" }, { "var": "tax" }] }
```

**Allowlist test.**

```json
{ "in": [{ "var": "country" }, ["US", "CA", "GB", "DE"]] }
```

**Filter the line items array down to taxable rows.**

```json
{ "filter": [
  { "var": "line_items" },
  { "==": [{ "var": "taxable" }, true] }
] }
```

## What Semantius adds

Semantius extends standard JsonLogic with reserved variables (`$today`, `$now`, `$user_id`, `$old`) and platform operators (`let`, `set_record`, `has_permission`, `require_permission`, `is_raci_actor`, `has_consultation`, `value_changed`, `concat`, `is_match`, `throw_error`). The extensions are scoped to the request the rule runs in and have no analog upstream. See [JsonLogic extensions](https://www.semantius.com/docs/business-logic/extensions.md) for the full list.

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## Related

- [Business Logic](https://www.semantius.com/docs/business-logic.md)
- [Extensions](https://www.semantius.com/docs/business-logic/extensions.md)
- [Docs home](https://www.semantius.com/docs.md)
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