Obvious JSON Parser

This lets you get JSON values and ignore arbitrary trailing text.

Available on npm and github

const obviously = require('obvious-json');

obviously.parse('{"a": 5}this-text-is-ignored'); // { a: 5 }
obviously.parse('{"a": 5}{"b": 6}');             // { a: 5 }

You can use this for streaming, or within other parsers

If you're streaming JSON objects, you can use parsev to get the length of the value that was parsed:

obviously.parsev('{"a": 5}{"b": 6}'); // { data: { a: 5 }, length: 8 }

It follows that you can use this within a lexer or a parser library that uses composition.

There's some backstory behind this

This package is called obvious-json because I think this behaviour is obvious.

Let me explain;

Any JSON value has an unambiguous ending. Therefore, it should be possible to parse a valid JSON object in any string when the starting position of this JSON object is known. That means you can have any garbage, or another JSON object, trailing a valid JSON object and still parse it.

The JSON parser built into javascript will even report an error with the end position included when there's trailing text, but I didn't want to resort to this nonsense. Why this simple feature doesn't exist in the builtin parser is beyond what my imagination can conjure.

Any alternate JSON implementation I found, such as streaming parsers, either didn't have an obvious way to get just a single value or had an entirely different purpose such as preservation of comments.

Contributing

If obvious-json doesn't do something it obviously should, then PRs to correct these are welcome. Here are some known issues:

• Parser context expects string input, so it doesn't work for asynchronous streaming. A change to add this would be considered in-scope as long as it doesn't break the existing interface.
• Numbers with an exponential component are calculated with floating-point error (ex: 1.1e-1 = 0.11000000000000001). The builtin JSON parser would report 0.11 exactly.
• This has only been tested as a CJS module. If anyone tries it as an ESM module and it works please close this issue. If it does not work, fixing that would be a welcome change.
• This parser is slower than the built-in JSON parser, so a C++ implementation might be worthwhile.
• Benchmark is not very thorough; it could measure each parser separately.

Benchmark

You can compare with the native JSON parser by running:

node node_modules/obvious-json/benchmark

This was the output I got when I tried it. It shows about a 7x performance decrease from the built-in parser.

native  parser: 208.0862100124359ms
obvious parser: 1491.626119852066ms
difference: 716.831%

The test is done using an array of 100000 objects, each containing 7 numbers with fractional and exponential components, 7 strings each containing a quote escape and two unicode escapes, and 7 of the value true.

Commenting out all but the numeric values brings the decrease down from 7x ro 5.5x. Contributions of other performance findings or benchmarks are very welcome.