How to convert JSON to JavaScript object literals and back

Every JSON document is already a valid JavaScript expression. You can paste JSON into a const config = assignment and it runs. The interesting work is the reverse: real JavaScript object literals are full of things JSON forbids, and a converter that guesses what they meant loses data while reporting success.

Using the JSON to JavaScript converter

Pick a direction and press Convert. Writing to JavaScript, an object key that is a valid identifier is written bare, so "name" becomes name, while a key like "display name" stays quoted because writing it bare would change its spelling. An empty object or array stays on one line. Numbers travel through the token stream rather than through JSON.parse, so a twenty digit integer comes back with all twenty digits.

What the reader accepts

Reading JavaScript back into JSON, the tool understands the object literals you would actually paste out of a config file: unquoted keys, single quoted strings, // and /* */ comments, trailing commas, and an optional const, let or var name at the front with a semicolon at the end. A template literal without interpolation is accepted too, since it holds plain text.

A number spelling JSON rejects is normalized to its exact JSON form:

const numbers = { a: .5, b: 5., c: 5.e3 }

{
  "a": 0.5,
  "b": 5,
  "c": 5e3
}

What is refused, and why

Anything JSON cannot hold is refused by name rather than guessed at. undefined, NaN and Infinity have no JSON number or value, so they are refused. So are hex, octal and binary numbers, underscore digit separators like 1_000, and a leading zero, all of which would change meaning if JSON read them. A regular expression, a function, a bare word, and ${...} template interpolation are refused as well, because those are code, not data.

Two refusals deserve a closer look. A key that appears twice in one object is refused, since JSON has no way to hold both values under one name. And a __proto__ key whose value is an object or null is refused, because in a JavaScript object literal that spelling does not create a property. It sets the prototype of the new object, so converting it to JSON with an ordinary __proto__ key would invent a property the original code never had.

Why undefined is not turned into null by default

JSON has no undefined, so a converter has to do something. Writing null silently changes what the data meant: an undefined in a config object usually means a field is not configured yet, which is not the same as a null the code later checks for. The tool refuses by default, and the Convert undefined to null option exists for when you genuinely want that meaning.

Round tripping

Converting a document back to the other format is the real test of whether the quoting and refusal rules are right. An object literal with unquoted keys, single quotes and a trailing comma comes back as JSON with every key quoted and every digit intact, and converting that JSON forward again returns the same object. A wrong rule usually only shows up when you convert back and compare.