json-logic-js

json-logic-js is the JavaScript implementation of JsonLogic, a shared rule format that expresses conditional logic as plain JSON so the same rule set can run identically across languages, environments, and databases. A rule is a single-key object mapping an operator to its arguments, which makes rules simple enough to author by hand, generate from a UI, store next to a database record, or send over an API — without duplicating business logic in every language that needs to evaluate it.

The library ships as a single dependency-free UMD module (logic.js) that runs unmodified in the browser, in Node via CommonJS, or through an AMD loader. It implements the full core JsonLogic operator set — comparisons, boolean logic, arithmetic, string operations, and array operations like map, filter, reduce, all, some, and none — plus data access via var, validation helpers (missing, missing_some), and utilities for introspecting rules (uses_data, rule_like) or registering custom operators at runtime.

What You Get

• A single self-contained logic.js UMD module with zero runtime dependencies, usable via npm, a <script> tag, or an AMD loader
• A complete JsonLogic operator set out of the box: comparisons, boolean logic, arithmetic, string operations, and array operations (map, filter, reduce, all, some, none)
• var, missing, and missing_some operators for reading and validating fields on a data object supplied at evaluation time
• add_operation/rm_operation for registering or removing custom operators at runtime, plus rule_like/uses_data for pattern-matching and introspecting rules before running them

Common Use Cases

• Storing user-configurable business rules (pricing, eligibility, feature flags) as JSON in a database and evaluating them at request time
• Sharing a single validation or eligibility rule set between a Node backend and a browser front-end without reimplementing the logic twice
• Building a visual rule builder UI where non-developers compose conditions that compile directly to JsonLogic
• Running the same rule against different data shapes to unit-test business logic without redeploying code

Under The Hood

Architecture

The entire library lives in one UMD-wrapped module built around a single recursive dispatcher, jsonLogic.apply(logic, data). Most operators are simple functions in a flat operations lookup table that receive their evaluated arguments and the data object as this; a handful of operators that need to control their own recursion or short-circuit (if, and, or, filter, map, reduce, all, none, some) are special-cased directly inside apply before falling through to the generic depth-first evaluation path. This keeps the control flow centralized in one function rather than split across files, so extending the language for anything beyond a plain value-in/value-out operator means editing that dispatcher directly rather than composing independent pieces — a workable design at this size, but one where every feature ultimately funnels through a single, growing conditional chain.

Tech Stack

Plain, dependency-free JavaScript targeting broad compatibility: the module uses the UMD pattern to support CommonJS (module.exports), AMD (define), and a global window.jsonLogic fallback from one file, with package.json pointing main directly at the unbundled logic.js — there is no build or bundling step in the publish path. Development tooling is limited to ESLint (Google config) for linting and QUnit for the test runner; there are no runtime dependencies and no TypeScript, so consumers get JavaScript with community-maintained type definitions rather than first-party types.

Code Quality

Testing is thorough for the library’s scope: the QUnit suite in tests/tests.js downloads a shared, cross-language conformance test set (tests.json, rule_like.json) from jsonlogic.com — the same fixtures used to validate JsonLogic ports in other languages — and layers on hand-written edge-case tests for falsy data, custom operator registration, and error paths. Error handling is explicit: unrecognized operators throw a descriptive Error rather than failing silently. No CI workflow configuration is present in the repository, and there is no static type system, so correctness relies on the conformance suite plus ESLint rather than compiler-enforced guarantees.

API Design The public surface is intentionally tiny: one entry point, jsonLogic.apply(rule, data), evaluates any rule against any data object, with syntactic sugar that lets unary operators skip the surrounding array ({"var": "a"} instead of {"var": ["a"]}). Getting started requires no setup beyond importing the module and calling apply — there’s no configuration object, no class to instantiate, and no boilerplate. add_operation extends the language with the same low ceremony, and the operator vocabulary mirrors the shared JsonLogic spec documented (with a live playground) at jsonlogic.com, which lowers the learning curve for anyone who has seen JsonLogic in another language.