json-ast

A tolerant JSON parser

Features

The original code was developed by Vlad Trushin. Breaking modifications were made by Romain Gaucher to create a less strict JSON parser. Additionally, a more typical interaction with the AST has been implemented.

Current modifications and features as of 2.1.6 include:

• Creation of a JsonDocument root node and more formal AST structure
• Support for inline comments
• Support for multi-line comments
• Support for trailing commas and many consecutive commas
• Include visitor pattern to visit the AST
• Include a limited error-recovery mode trying to catch (when junker set to true):
  • unclosed objects or arrays
  • too many closing braces or brackets
  • automatic comma injection
  • support for unquoted keys
• Conversion to a native JavaScript object from JsonNode

Basic examples are available to show how to use this package.

JSONish

The JSON parser accepts a superset of the JSON language:

// some comment
{
  "key1": "value1", // some other comments
  "key2": "value2",
  ,
  ,
  /*
    Oh dear! It's important to put this here.
    And we love commas too!
    And we're missing the closing brace...
  */

Install

npm install json-ast

Structure of the AST

As of 2.1.0, the AST is defined with the following types:

[JsonNode] // Essentially an abstract class
  position: [Position]
 
[JsonDocument] extends [JsonNode]
  child: [?]*
  comments: [JsonComment]*
 
[JsonValue] extends [JsonNode]
  value: [?]
 
[JsonObject] extends [JsonNode]
  properties: [JsonProperty]*
  comments: [JsonComment]*
 
[JsonProperty] extends [JsonNode]
  key: [JsonKey]
  value: [?]*
 
[JsonKey] extends [JsonValue]
 
[JsonArray]
  items: *
  comments: [JsonComment]*
 
[JsonComment] extends [JsonValue]
[JsonString] extends [JsonValue]
[JsonNumber] extends [JsonValue]
[JsonTrue] extends [JsonValue]
[JsonFalse] extends [JsonValue]
[JsonNumber] extends [JsonValue]

All the types exists in src/ast.js.

API

import {parse, Visitor, AST} from 'json-ast';
 
// The visitor can stop at any time by assigning `Visitor.stop = true`
class MyVisitor extends Visitor {
  constructor() {
    super();
    this.comments = [];
  }
 
  comment(commentNode) {
    this.comments.push(commentNode.value);
  }
};
 
const JSON_BUFFER = `// Some comment
{
  "key": "value"
`;
 
// `verbose` will include the position in each node
const ast = parse(JSON_BUFFER, {verbose: true, junker: true});
assert(ast instanceof AST.JsonDocument);
 
const visitor = new MyVisitor();
ast.visit(visitor);
assert.deepEqual(visitor.comments, [" Some comment"]);
 
// One can also the `JsonNode.toJSON` static method to convert to a JavaScript object
const obj = JsonNode.toJSON(ast);
assert(obj.key === 'value');

Parsing Options

The second argument of the parse function takes an object with the following settings:

• verbose: include positions in each AST node, true by default
• junker: enables an error recovery mode, false by default

License

MIT Vlad Trushin and Romain Gaucher