JSON Formatter

What JSON actually is

JSON (JavaScript Object Notation) is a text interchange format defined by RFC 8259. It has exactly six data types: object, array, string, number, boolean, and null. Despite the name, JSON is not a subset of JavaScript syntax — it is its own, stricter grammar.

Strings must use double quotes; object keys must be quoted; there are no comments, no trailing commas, and no bare NaN or Infinity. That strictness is the point: any compliant parser, in any language, reads the same bytes the same way.

• {} object and [] array — named and ordered collections
• Numbers are IEEE 754 double precision — integers lose precision past 2⁵³
• true, false, and null are the only literals

Valid JSON is not always valid JavaScript

A common trap: text that runs fine in a .js file makes JSON.parse throw. The three usual suspects are single-quoted strings, unquoted keys, and trailing commas — all legal JavaScript, all illegal JSON.

There is also the subtler U+2028 / U+2029 case: these line separators are valid inside JSON strings but were invalid in JavaScript string literals before ES2019. This is why you should never eval() JSON — always parse it.

// Valid JavaScript, INVALID JSON — JSON.parse throws on all three:
{ 'key': 'value', }   // single quotes + trailing comma
{ key: "value" }      // unquoted key

How pretty-printing works under the hood

Indentation comes from JSON.stringify(value, replacer, space). The third argument controls spacing — pass 2 for two spaces, or "\t" for tabs. The second argument is a replacer: a function to filter or transform values, or an array of keys to whitelist.

For very large payloads (multi-megabyte logs), formatting has a real memory cost because the entire structure is serialized into a new string. Validate first, format second, and reach for a streaming parser when input is huge.