llm-json-compact

Compact JSON into a table-like structure that removes repeated keys and is easier for LLMs to consume with fewer tokens.

Instead of repeating field names on every object, llm-json-compact lifts shared keys into a header row and keeps only the values in each row.

Why

Raw JSON is verbose for LLM input, especially for arrays of objects.

[
  { "name": "John", "age": 30, "city": "New York" },
  { "name": "Jane", "city": "Boston" }
]

With llm-json-compact:

[["name","age","city"],["John",30,"New York"],["Jane",,"Boston"]]

Install

npm install llm-json-compact

Quick Start

import { jsonCompact, stringifyCompact } from "llm-json-compact";

const compacted = jsonCompact([
  { name: "John", age: 30, city: "New York" },
  { name: "Jane", city: "Boston" },
]);

console.log(compacted);
// [
//   ["name", "age", "city"],
//   ["John", 30, "New York"],
//   ["Jane", undefined, "Boston"]
// ]

console.log(stringifyCompact(compacted));
// [["name","age","city"],["John",30,"New York"],["Jane",,"Boston"]]

API

jsonCompact(input)

Compacts a plain object or an array of plain objects into a nested array structure.

stringifyCompact(compacted)

Serializes compacted output into a shorter string form for LLM prompts.

Rules:

• Middle undefined values are rendered as empty slots: ["Jane",,"Boston"]
• Trailing undefined values are omitted
• null stays null

Output Rules

• A plain object becomes a two-row table: headers + values
• An array of objects becomes one header row plus one row per object
• A nested object becomes a single compact column like address[city,zip]
• An array of objects inside a field becomes a compact column like addresses[city,street]
• Scalar arrays are kept as-is
• New keys discovered in later rows are appended to the header row

Examples

Plain object

jsonCompact({
  name: "John",
  age: 30,
  city: "New York",
});

// [
//   ["name", "age", "city"],
//   ["John", 30, "New York"]
// ]

Nested object

jsonCompact({
  name: "John",
  age: 30,
  address: {
    city: "New York",
    zip: "10001",
  },
});

// [
//   ["name", "age", "address[city,zip]"],
//   ["John", 30, ["New York", "10001"]]
// ]

Array of objects

jsonCompact([
  { name: "John", age: 30, hobbies: ["reading", "traveling"] },
  { name: "Jane", age: 25, hobbies: ["cooking"], city: "New York" },
]);

// [
//   ["name", "age", "hobbies", "city"],
//   ["John", 30, ["reading", "traveling"]],
//   ["Jane", 25, ["cooking"], "New York"]
// ]

Nested array of objects

jsonCompact({
  name: "John",
  age: 30,
  addresses: [
    { city: "New York", street: "1225 Hanover Street" },
    { city: "Los Angeles", street: "456 Sunset Boulevard" },
  ],
});

// [
//   ["name", "age", "addresses[city,street]"],
//   [
//     "John",
//     30,
//     [
//       ["New York", "1225 Hanover Street"],
//       ["Los Angeles", "456 Sunset Boulevard"]
//     ]
//   ]
// ]

Limitations

• Input must be a plain object or an array of plain objects
• Empty arrays are inferred with a small heuristic, so field naming can affect whether an empty array is treated as scalar or object-like
• The format is intentionally compact, not strict JSON schema
• This is optimized for LLM consumption, not lossless typed serialization

Development

npm test
npm run build