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Introduction

Datadance is an open-source JSON data transformation tool. It accepts JSON input along with user-defined transformation rules expressed using MozJexl expressions and produces transformed JSON output.

Datadance is designed for ETL pipeline scenarios where executing third-party or user-provided code is both an operational and security challenge. Instead of running arbitrary code, user logic is expressed as declarative transforms — easy to write, maintain, and audit.

Features​

  • Safe execution — No arbitrary code execution. Transforms are declarative expressions evaluated by MozJexl.
  • Rich transform library — 60+ built-in transforms for strings, arrays, objects, numbers, dates, and more.
  • Multiple merge modes — overwrite, preserve, and transforms_only to control how output merges with input.
  • Nested transformations — Apply transforms recursively on nested objects.
  • Temporary fields — Use _-prefixed fields for intermediate values without polluting the output.
  • Sub-transform blocks — Reusable helper blocks (_$-prefixed) within transform definitions.
  • DDS syntax — Write transforms in a clean YAML-like syntax instead of JSON.
  • CLI tool — Run transforms from the command line.
  • REST API — Process transforms via HTTP endpoints.
  • Interactive Playground — Experiment with transforms in your browser at the Playground.

How it works​

Datadance takes your Input JSON, applies a series of transforms, and produces an Output JSON.

{
  "name": {
    "first": "Malory",
    "last": "Archer"
  },
  "exes": [
    "Nikolai Jakov",
    "Len Trexler",
    "Burt Reynolds"
  ],
  "lastEx": 2
}
Input vs Derived
  • input — Immutable reference to the original data. input.lastEx always returns 2.
  • derived — Mutable state that accumulates changes. derived.lastEx returns 7 after the first transform runs.

Usage​

import { transform } from "datadance";

const result = await transform({
  input: { x: 2 },
  transforms: [{ x: "input.x + 8" }],
  settings: { merge_method: "overwrite" },
});

console.log(result); // { x: 10 }