JSON parser written in Typescript
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
``
npm i ts-jsonparse
`Javascript
import { Parser } from 'ts-jsonparse';
const parser = new Parser(text);
const obj = parser.parse();
`
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.
The job of the Lexer is to break the parts of the string into tokens. For JSON we look for the following tokens.
`Javascript`
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',
}
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
``
The JSONBuilder walks the AST and builds the Javascript represntation of the JSON string.