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JAC (JSON as CSV) Format

The JAC format is a flexible CSV expression of JSON files that makes it easy to convert to and from JSON and CSV file formats, while giving application developers a lot of flexibility to customize how easy it is to modify the CSV file for end users. Any JSON object can be represented as a JAC CSV.

.jac.csv files are always valid CSVs.

An example JAC file is shown below:

path . name dogs.0 dogs.1
myName John
friends.0 Stacy Rufus
friends.1 Paul Mr. Fluffs Whimpers

When this file is converted into JSON, it becomes:

  "myName": "John",
  "friends": [
      "name": "Stacy",
      "dogs": ["Rufus"]
      "name": "Paul",
      "dogs": ["Mr. Fluffs", "Whimpers"]


CSVs can be much easier for end users, especially if the end users are not programmers. But when converting a file into a CSV, I felt it was annoying to have to come up with some specification and maintain that specification against an internal JSON format.

The JAC format allows you to easily define a conversion from your JSON format to your CSV format with enough flexibility such that nested JSON structures are still editable, and unimportant details are not emphasized (by being grouped in a cell).

The JAC Format for the Universal Data Tool to convert the JSON representation to and from readable CSV representations. Here's an example of a .jac.csv file output by the Universal Data Tool.

Usage with Javascript

npm install jac-format

const JAC = require("jac-format")

let csvString = JAC.toCSV(
    fruit: [{ name: "apple" }, { name: "lemon" }],
    rows: ["fruit.*"], // optional
    columns: ["name"], // optional
// "path,name\r\nfruit.0,apple\r\nfruit.1,lemon"
// ┌───┬───────────┬─────────┐
// │   │     A     │    B    │
// ├───┼───────────┼─────────┤
// │ 1 │  'path'   │ 'name'  │
// │ 2 │ 'fruit.0' │ 'apple' │
// │ 3 │ 'fruit.1' │ 'lemon' │
// └───┴───────────┴─────────┘

// You can also use this
// > { "fruit": [{ "name": "apple" }, { "name": "lemon" }] }

// JAC.fromCSV === JAC.toJSON
// JAC.fromJSON === JAC.toCSV

Usage with Python

pip install jac_format

import jac_format as jac

csv_string = jac.to_csv(
    "fruit": [{ "name": "apple" }, { "name": "lemon" }],
  rows=["fruit.0", "fruit.1"],

# > csv_string
# "path,name\r\nfruit.0,apple\r\nfruit.1,lemon"
# ┌───┬───────────┬─────────┐
# │   │     A     │    B    │
# ├───┼───────────┼─────────┤
# │ 1 │  'path'   │ 'name'  │
# │ 2 │ 'fruit.0' │ 'apple' │
# │ 3 │ 'fruit.1' │ 'lemon' │
# └───┴───────────┴─────────┘

# { "fruit": [{ "name": "apple" }, { "name": "lemon" }] }


  • JAC CSV files are valid RFC4180 CSVs
  • jac_csv_path or path is the first column, first cell
  • The first column contains the first path segment (except for the jac_csv_path cell)
  • The first row (header) contains the second path segment (except for the jac_csv_path cell)
  • Each value cell of a JAC CSV can be
      1. an empty cell
      1. a string
      1. a JSON object
      1. null
      1. a number
      1. a JSON array
  • Columns right of the "path" column are applied in order from left to right. Each row creates an object. This object is then set at the path of the first column. (except in column-first mode)
  • A path can be traversed with either square bracket notation or dot notation
  • In dot notation, the usage of a number indicates the index of an array (a["1"].0 is equivalent to a["1"][0])
  • If an array has undefined values, those values are set to null
  • A value cell's path is constructed by taking the leftmost cell of of a row (in the path column) and appending the topmost header to it

Automatic Indexing with "*"

Automatic indexing makes it easier to add and delete rows because index numbers don't need to be adjusted.

These tables are equivalent when converted to JSON:

path . name dogs.* dogs.*
myName John
friends.* Stacy Rufus
friends.* Paul Mr. Fluffs Whimpers
path . name dogs.0 dogs.1
myName John
friends.0 Stacy Rufus
friends.1 Paul Mr. Fluffs Whimpers

If "*" are replaced by the smallest index in the path segment that's not already taken. There are two appropriate syntaxes, "[*]" or ".*". For a row, only the path segments in the row are considered (i.e. the header is converted into indicies without any information from the path column).

You can also use the * to refer to the last object created matching the prefix preceding the star. The example below is equivalent to the two tables above.

path . name
myName John
friends.* Stacy
friends.*.dogs.0 Rufus
friends.* Paul
friends.*.dogs.0 Mr. Fluffs
friends.*.dogs.1 Whimpers

Column-First Mode

If "path (column first)" is in the first cell of the first row, then columns are applied as the first segment of the path, and rows are applied as the second segment. This is great for data that that grows over time.

path (column first) . weeks.0 weeks.1 weeks.2
A.pr_count 3 2 1
B.pr_count 2 1 3

Pros & Cons

  1. The flexibility of the JAC CSV format allows applications that output JAC CSV to give the user CSV data in a "flattening" that is most convenient for the application i.e. Columns can be created to make it easy for the user to modify the data.
  2. As a result of the flexibility in the JAC CSV format, one JSON file can have almost an infinite amount of CSV variations.
  3. Column order matters because it determines how the CSV is merged back into JSON




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