showwhat

showwhat is a definition format, rule engine, and visual tooling for feature flags and config.

• Portable format — YAML/JSON definitions you can store in files, APIs, databases, or version control
• In-app evaluation — resolve flags and config inside your own application
• Extensible rule engine — schema-validated, fail-closed, with custom condition types
• Optional visual tooling — author and test definitions in the configurator

See Comparison for how this differs from hosted 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. Use definitions for boolean flags, config values, or structured objects. A variation with no conditions always matches, so use it as the 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({
  keys: ["checkout_v2"],
  context: { env: "prod" },
  options: { data },
});

const entry = result["checkout_v2"];
if (!entry.success) {
  console.log(entry.error); // ShowwhatError
} else {
  console.log(entry.value); // true
}

The context object tells showwhat about the current environment. The data option provides the definitions. The result is a Resolutions record keyed by definition name, with each entry is either a successful Resolution (success: true) or a ResolutionError (success: false).

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