jsonous

If you're building a web application, you're eventually going to need a reliable way to convert JSON into type that you can use. JSON.parse will only get you so far.

A better solution allows you to verify the structure of the source data while converting it into the types you need to work with. It handles variations in content structure. In a typed environment, it preserves your types so you don't have to pass any types around. Errors are handled gracefully using a Result type.

This library, heavily inspired by Elm's JSON Decoder, is just such a solution.

install

npm install --save jsonous

yarn add jsonous

usage

primitives

There are several decoders for handling JSON primitives; strings, numbers, booleans, and arrays (more on objects later)

string.decodeJson('"foo"'); // --> Ok("foo")
string.decodeJson('42'); // --> Err("42 is not a string")

number.decodeJson('42'); // --> Ok(42)
number.decodeJson('"foo"'); // --> Err("foo is not a number")

boolean.decodeJson('true'); // --> Ok(true)
boolean.decodeJson('"foo"'); // --> Err("foo is not a boolean")

arrays

The array primitive applies another decoder to an array of values.

array(string).decodeJson('["foo", "bar"]'); // --> Ok(["foo", "bar"])
array(string).decodeJson('["foo", 42]'); // --> Err("42 is not a string")

objects

The field and at decoders are used to extract values from objects.

field('bar', string).decodeAny({ bar: 'baz' }); // --> Ok('baz')
at(['foo', 0, 'bar'], number).decodeAny({ foo: [{ bar: 42 }] }); // --> Ok(42)

Object decoders can be chained together to build more complex data structures.

// prettier-ignore
field('userId', number).andThen(id =>
field('emailAddress', string).andThen(email =>
succeed({ id, email })))
.decodeAny({ userId: 213, emailAddress: 'foo@example.com' })
// --> Ok({ id: 213, email: 'foo@example.com'})

Of course, your code editor may try to reformat this code.

field('userId', number)
  .andThen((id) =>
    field('emailAddress', string).andThen((email) => succeed({ id, email }))
  )
  .decodeAny({ userId: 213, emailAddress: 'foo@example.com' });
// --> Ok({ id: 213, email: 'foo@example.com'})

For an object of any moderate complexity, this nesting is indistinguishable from callback hell.

To combat this, decoders have a method named assign. It enacapsulates this pattern of building objects such that objects can be built incrementally, without nesting and without losing type safety.

The previous example would look like this using assign:

succeed({})
  .assign('id', field('userId', number))
  .assign('email', field('emailAddress', string))
  .decodeAny({ userId: 213, emailAddress: 'foo@example.com' });
// --> Ok({ id: 213, email: 'foo@example.com' })

The best strategy for handling object construction with decoders is to compose complex decoders from smaller, simpler decoders.

succeed({})
  .assign('user', field('user', userDecoder))
  .assign('courses', at(['student', 'courses'], array(courseDecoder)))
  .decodeJson('... some json ... ')

const userDecoder = //...
const courseDecoder = //...

docs

API