showwhat

An extensible schema and rule based evaluation engine for feature flags and configuration.

• Specification-first — definitions are YAML/JSON files that live in Git
• Zero infrastructure — no server, database, or vendor to operate
• Extensible — add custom condition types, bring your own data source

See Comparison for how this differs from feature flag platforms.

Quick Start

Install the core package:

# Node.js
npm install showwhat
pnpm add showwhat
yarn add showwhat

# Other runtimes
bun add showwhat
deno install npm:showwhat

Your first definition

Create a file with your definitions in YAML or JSON:

flags.yaml

definitions:
  checkout_v2:
    variations:
      - value: true
        conditions:
          - type: env
            value: prod
      - value: false

flags.json

{
  "definitions": {
    "checkout_v2": {
      "variations": [
        { "value": true, "conditions": [{ "type": "env", "value": "prod" }] },
        { "value": false }
      ]
    }
  }
}

Each definition has variations evaluated top-to-bottom. The first variation whose conditions all match wins. A variation with no conditions always matches (use it as a default).

Resolve a definition

import { showwhat, MemoryData } from "showwhat";

const data = await MemoryData.fromYaml(`
definitions:
  checkout_v2:
    variations:
      - value: true
        conditions:
          - type: env
            value: prod
      - value: false
`);

const result = await showwhat({
  key: "checkout_v2",
  context: { env: "prod" },
  options: { data },
});

console.log(result.value); // true

The context object tells showwhat about the current environment. The data option provides the definitions.

Next steps

• Definitions — how definitions are structured
• Conditions — all built-in condition types
• Context — the context object
• Data Sources — built-in MemoryData source
• Custom Data Sources — filesystem, HTTP, and other implementations
• OpenFeature — use showwhat with the OpenFeature SDK