ts-jsonparse

Overview

I decided one afternoon to write a JSON parser to improve my Typescript skills and reinforce what I learned from Ruslan Spivak's excellent blog on Let's Build a Simple Interpreter. This is a Typescript implementation of a simple JSON parser. It does not handle exponention numbers or backslashes '' in strings.

I used the JSON Interchange Standard - 2nd addition as a reference for the implementation

Installation

npm i ts-jsonparse

To Use

import { Parser } from 'ts-jsonparse';

const parser = new Parser(text);
const obj = parser.parse();

Background

The JSON parser is divided into 3 parts:

• Lexer - tokenizes the JSON string
• Parser - Builds Abstract Syntax Tree
• JSONBuilder - Walks the AST and builds a Javascript representation of the JSON.

Lexer

The job of the Lexer is to break the parts of the string into tokens. For JSON we look for the following tokens.

export enum eTokens {
  BEGIN_ARRAY = '[', // left square bracket
  BEGIN_OBJECT = '{', // left curly bracket
  END_ARRAY = ']', // right square bracket
  END_OBJECT = '}', // right curly bracket
  COLON = ':',
  COMMA = ',',
  TRUE = 'TRUE',
  FALSE = 'FALSE',
  NULL = 'NULL',
  NUMBER = 'NUMBER',
  STRING = 'STRING',
  EOF = 'EOF',
}

Parser

The goal of the parser is to find structure in the stream of tokens. I choose to implement an Abstract Syntax Tree (AST) to represent the JSON string. I choose for the AST to include 4 Nodes:

jObject - to represent the object
jArray - to represent the array
jNameValue - to represent a name - value pair
jPrimative - to represent a number, string, true, false, or null

The Parser implements the following JSON Grammer to build the AST.

value : object
| array
| NUMBER
| STRING
| TRUE
| FALSE
| NULL

object : BEGIN_OBJECT name_value_list END_OBJECT

array : BEGIN_ARRAY value_list END_ARRAY

name_value_list: name_value
| name_value COMMA name_value_list

name_value: string COLON value

value_list: value
| value COMMA value_list

JSONBuilder

The JSONBuilder walks the AST and builds the Javascript represntation of the JSON string.