polite-json

This is a Node.js library for getting nicer errors out of JSON.parse(), including context and position of the parse errors.

It also preserves the newline and indentation styles of the JSON data, by putting them in the object or array in the Symbol.for('indent') and Symbol.for('newline') properties, so that if you stringify() the object you parsed, it'll be formatted with the same indentation and newlines (in most cases, it's not a super clever streaming in-place editor, it just uses the first line break and indentation it sees).

Install

$ npm install --save polite-json

Example

import { parse, parseNoExceptions, stringify } from 'polite-json'
// or: const { parse, stringify } = require('polite-json')

parse('"foo"') // returns the string 'foo'
const obj = parse('{\r\n\t"hello": "world"\r\n}') // { hello: 'world' }
stringify(obj) // stringifies with \r\n line breaks, \t indentation
parse('garbage') // more useful error message
parseNoExceptions('garbage') // no error, just undefined

Features

• Like JSON.parse, but the errors are better, and style choices are (mostly) preserved.
• Strips a leading byte-order-mark that you sometimes get reading files.
• Has a noExceptions method that returns undefined rather than throwing.
• Attaches the newline character(s) used to the Symbol.for('newline') property on objects and arrays.
• Attaches the indentation character(s) used to the Symbol.for('indent') property on objects and arrays.

Indentation

Indentation and newline information is saved on the Symbol.for('indent') and Symbol.for('newline') properties, respectively. This is usually not a problem for JSON objects, since JSON ignores symbols, but if you are using them for some other purpose, it could cause weird behavior.

Indentation is determined by looking at the whitespace between the initial { and [ and the character that follows it. If you have lots of weird inconsistent indentation, then it won't track that or give you any way to preserve it. Whether this is a bug or a feature is debatable ;)

API

parse(txt, reviver = null, context = 20)

Works just like JSON.parse, but will include a bit more information when an error happens, and attaches a Symbol.for('indent') and Symbol.for('newline') on objects and arrays. This throws a JSONParseError.

parseNoExceptions(txt, reviver = null)

Works just like JSON.parse, but will return undefined rather than throwing an error.

class JSONParseError(er, text, context = 20, caller = null)

Extends the JavaScript SyntaxError class to parse the message and provide better metadata.

Pass in the error thrown by the built-in JSON.parse, and the text being parsed, and it'll parse out the bits needed to be helpful.

context defaults to 20.

The er parameter is attached as the JSONParseError.cause property. This is used to provide a reference to the original system error that was raised by JSON.parse().

Set a caller function to trim internal implementation details out of the stack trace. When calling parseJson, this is set to the parseJson function. If not set, then the constructor defaults to itself, so the stack trace will point to the spot where you call new JSONParseError.

stringify(obj, reviver = undefined, indent = undefined)

If the object was parsed using the parse() methods in this library, then it will default to using the same indentation and newline strings that were detected, if relevant.

If indent is set, then the stored formatting information is ignored, and \n newlines will be used.

A final newline will be appended if indentation is used.