A loss-less JSON parser

Another JSON parser??

All of the JSON parsers I found focussed on producing natural JavaScript data structures from JSON, instead of correctly modelling the JSON document in Javascript.

What that means is that the line and column locations of elements in JSON documents is lost once a syntactically correct document is produced.

This, in turn, prevents good schema based editors from working easily. Because schema parsers tend to do their work on natural Javascript structures like Javascript Objects or Arrays.

So this parser is about that. It does not automatically return Javascript Objects or Arrays. Instead it produces an object that can then be turned into it's natural Javascript form.

For example:

{
  "a": "1",
  "b": {
    "c": "12",
    "d": "13"
  } 
}

Becomes, in print form:

ObjectValue {
  type: 'object',
  elements:
   { a: { [String: '1'] line: 1, column: 7, type: 'string' },
     b:
      ObjectValue { type: 'object', elements: {
        c: { [String: '12'] line: 3, column 9, type: 'string' },
        d: { [String: '13'] line: 4, column 9, type: 'string' }
      }, line: 2, column: 7 } },
  line: 0,
  column: 0 }

Example

How to program with it? Well, for example, in Node:

const jsonParser = require("@nicferrier/json-parser");

const jsonDocument = jsonParser(`{
   "a": "10",
   "b": "20",
   "c": [
      "hello",
      "world",
      "goodbye"
   ]
}`);

const data = jsonDocument.valueOf();

data.a === "10";                          // natural object
jsonDocument.get("a") == "10";            // note the `==` to get effective equal
jsonDocument.get("a").valueOf() === "10"; // parser objects only

jsonDocument.get("a").line === 1;
jsonDocument.get("b").line === 2;
jsonDocument.get("b").column === 8;       // start of the quote for "20"

data.c[1] === "world";
jsonDocument.get("c").get(1) == "world";

What about errors?

const jsonParser = require("@nicferrier/json-parser");

const bad = `{ true: "hello" }`;
try {
    jsonParser(bad);
}
catch (e) {
    assert.ok(e instanceof jsonParser.KeyTypeError);
    assert.ok(e.line === 0);
    assert.ok(e.column === 2);
}

Objects returned

JSON strings, including keys, are Javascript String objects with a type property of "string" and a line and column property.

JSON boolean types, which are the true and false identifiers in the Javascript language are supported as Boolean types but with the additional properties of line and column.

JSON object types, which are key: value sets are ObjectValue types which have the following methods:

• getKey(key) given a key in the map returns the token representation of the key; the token representation has the line and column proeprties on it

• get(key) given a key in the map, returns the representation of the value

• set(key, value) sets key to be value - but value and key must be parser types, not just any value

• valueOf() returns a Javascript native Object representation of this ObjectValue.

In addition the line and column properties are supported.

JSON array types, which are dynamically extendable lists of values, are ArrayValue types which have the following methods:

• get(index) returns the value stored at the index index

• push(element) pushes element onto the end of the array, but element should be a parser type, not a native Javascript type

• valueOf() returns a Javascript native Array representation of this ArrayValue.

In addition, line and column properties are supported as well as the length property which always states the count of the actual elements in the ArrayValue.

JSON numeric types are not currently supported by the parser.

What errors are produced?

All errors are extensions of ParserSntaxError which extends from SyntaxError which is a normal Javascript error.

All errors are maintained on the parser function for ease of import.

EOFError is thrown when there are expectations of input but none exists; for example:

{ "a"

will throw EOFError and give the line and column of the error, which is where the input runs out.

KeyError is thrown when a key is not properly terminated with a colon, for example:

{ "a" "hello"

The KeyError will point at the column starting "hello" because that is the token that invalidates our colon expectation.

KeyTypeError is thrown when a key is not the correct "string" type; for example:

{ true: "hello" }

throws a KeyTypeError at true because that is not a legal key value in JSON.

UnexpectedTypeError is thrown when a value is not followed by a comma or a legal ending token; for example when you forget the comma:

{
  "greeting": "hi"
  "farewall": "by-by"
}

There's no comma after the "hi" so the next token is a string... not what the parser expects so it throws the error.

Hacking

For details of how to hack on this see HACKING.md.