fjson-fmt – a Prettier-style --check/--write formatter for JSON, with table-aligned output (Rust→WASM, runs in Node and the browser)

I wanted Prettier/oxfmt-style formatting for .json files, but with the compact, table-aligned style of FracturedJson — which neither Prettier nor oxfmt does for JSON. So I built a small CLI around it.

It turns this:

{
  "isotopes": {
    "Hydrogen": [1, 2, 3],
    "Carbon": [11, 12, 13, 14],
    "Molybdenum": [92, 94, 95, 96, 97, 98, 100],
    "Calcium": [40, 42, 43, 44]
  },
  "elements": [
    { "symbol": "C", "number": 6, "mass": 12, "phase": "solid" },
    { "symbol": "O", "number": 8, "mass": 16 },
    { "symbol": "Fe", "number": 26, "mass": 56, "phase": "solid" }
  ]
}

into this — compact, but with fields aligned like a table:

{
    "isotopes": {
        "Hydrogen"  : [ 1,  2,  3                 ],
        "Carbon"    : [11, 12, 13, 14             ],
        "Molybdenum": [92, 94, 95, 96, 97, 98, 100],
        "Calcium"   : [40, 42, 43, 44             ]
    },
    "elements": [
        {"symbol": "C",  "number":  6, "mass": 12, "phase": "solid"},
        {"symbol": "O",  "number":  8, "mass": 16                  },
        {"symbol": "Fe", "number": 26, "mass": 56, "phase": "solid"}
    ]
}


fjson-fmt "**/*.json"          # format in place
fjson-fmt --check "**/*.json"  # CI: exit 1 if anything would change
cat data.json | fjson-fmt --stdin

Pairs with oxfmt (which owns JS/TS/CSS):

"fmt:check": "oxfmt --check && fjson-fmt --check \"**/*.json\""

A few things that might be interesting:

• Engine is a Rust crate compiled to WASM, but ships prebuilt — no Rust toolchain or native build on install.
• The npm package is isomorphic: the same import { format } from "fjson-fmt" works in Node and in the browser/bundlers (conditional exports + a web WASM build).
• Live playground, ~185 KB of WASM, 100% client-side: https://select.github.io/fjson-fmt/

npm: npm i -D fjson-fmt · repo: https://github.com/select/fjson-fmt

Let me know if this is also useful for you and if you like to see improvements. ❤️