json-decoder

TypeScript JSON Decoder: json-decoder

json-decoder is a type safe compositional JSON decoder for TypeScript. It is heavily inspired by Elm and ReasonML JSON decoders. The code is loosely based on ts.data.json but is a full rewrite, and does not rely on unsafe any type.

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Compositional decoding

The decoder comprises of small basic building blocks (listed below), that can be composed into JSON decoders of any complexity, including deeply nested structures, heterogenous arrays, etc. If a type can be expressed as TypeScript interface or type (including algebraic data types) - it can be safely decoded and type checked with json-decoder.

Install (npm or yarn)

  $> npm install json-decoder
  $> yarn add json-decoder

Basic decoders

Below is a list of basic decoders supplied with json-decoder:

• stringDecoder - decodes a string:

  const result: Result<string> = stringDecoder.decode("some string"); //Ok("some string");
  const result: Result<string> = stringDecoder.decode(123.45); //Err("string expected");
  

• numberDecoder - decodes a number:

  const result: Result<number> = numberDecoder.decode(123.45); //Ok(123.45);
  const result: Result<number> = numberDecoder.decode("some string"); //Err("number expected");
  

• boolDecoder - decodes a boolean:

  const result: Result<boolean> = boolDecoder.decode(true); //Ok(true);
  const result: Result<boolean> = boolDecoder.decode(null); //Err("bool expected");
  

• nullDecoder - decodes a null value:

  const result: Result<null> = nullDecoder.decode(null); //Ok(null);
  const result: Result<null> = boolDecoder.decode(false); //Err("null expected");
  

• undefinedDecoder - decodes an undefined value:

  const result: Result<undefined> = undefinedDecoder.decode(undefined); //Ok(undefined);
  const result: Result<undefined> = boolDecoder.decode(null); //Err("undefined expected");
  

• arrayDecoder<T>(decoder: Decoder<T>) - decodes an array, requires one parameter of array item decoder:

  const numberArrayDecoder = arrayDecoder(numberDecoder);
  const result: Result<number[]> = numberArrayDecoder.decode([1,2,3]); //Ok([1,2,3]);
  const result: Result<number[]> = numberArrayDecoder.decode("some string"); //Err("array expected");
  const result: Result<number[]> = numberArrayDecoder.decode([true, false, null]); //Err("array: number expected");
  

• objectDecoder<T>(decoderMap: DecoderMap<T>) - decodes an object, requires a decoder map parameter. Decoder map is a composition of decoders, one for each field of an object, that themselves can be object decoders if neccessary.

  type Pet = {name: string, age: number};
  const petDecoder = objectDecoder<Pet>({
    name: stringDecoder,
    age: numberDecoder,
  });
  const result: Result<Pet> = petDecoder.decode({name: "Varia", age: 0.5}); //Ok({name: "Varia", age: 0.5});
  const result: Result<Pet> = petDecoder.decode({name: "Varia", type: "cat"}); //Err("name: string expected");
  
  const petDecoder = objectDecoder<Pet>({
    name: stringDecoder,
    type: stringDecoder, //<-- error: field type is not defined in Pet
  });
  

• exactDecoder<T>(value: T) - decodes a value that is passed as a parameter. Any other value will result in Err:

  const catDecoder = exactDecoder("cat");
  const result: Result<"cat"> = catDecoder.decode("cat"); //Ok("cat");
  const result: Result<"cat"> = catDecoder.decode("dog"); //Err("cat expected");
  

• oneOfDecoders<T1|T2...Tn>(...decoders: Decoder<T1|T2...Tn>[]) - takes a number decoders as parameter and tries to decode a value with each in sequence, returns as soon as one succeeds, errors otherwise. Useful for algebraic data types.

  const catDecoder = exactDecoder("cat");
  const dogDecoder = exactDecoder("dog");
  const petDecoder = oneOfDecoders<"cat"|"dog"> = oneOfDecoders(catDecoder, dogDecoder);
  
  const result: Result<"cat"|"dog"> = petDecoder.decode("cat"); //Ok("cat");
  const result: Result<"cat"|"dog"> = petDecoder.decode("dog"); //Ok("dog");
  const result: Result<"cat"|"dog"> = petDecoder.decode("giraffe"); //Err("none of decoders matched");
  

• allOfDecoders(...decoders: Decoder<T1|T2...Tn>[]): Decoder<Tn> - takes a number decoders as parameter and tries to decode a value with each in sequence, all decoders have to succeed. If at leat one defocer fails - returns Err.

  const catDecoder = exactDecoder("cat");
  const result: Result<"cat"> = allOfDecoders(stringSecoder, catDecoder); //Ok("cat")
  

Type inference

Type works both ways - not only you can specify type for a decoder, it is also possible to infer the type from an existing decoder, particularly useful for composition of decoders:

type Number = DecoderType<typeof numberDecoder>; //number
const someDecoder = objectDecoder({
  field1: stringDecoder,
  field2: numberDecoder,
  field3: arrayDecoder(numberDecoder)
});
type Some = DecoderType<typeof someDecoder>; // {field1: string, field2: number, field3: number[] }
const some: Some = await someDecoder.decodeAsync({...});

const stringOrNumberDecoder = oneOfDecoders<string |number>(stringDecoder, numberDecoder);
type StringOrNumber = DecoderType<typeof stringOrNumberDecoder>; //string | number

API

Each decoder has the following methods:

• decode(json:unknown): Result<T> - attempts to decode a value of unknown type. Returns Ok<T> if succesful, Err<T> otherwise.

• decodeAsync(json:unknown): Promise<T> - Returns a Promise<T> that attempts to decode a value of unknown type. Resolves with T if succesful, rejects Error{message:string} otherwise. A typical usage of this would be in an async function context:

  const getPet = async (): Promise<Pet> => {
    const result = await fetch("http://some.pet.api/cat/1");
    const pet: Pet = await petDecoder.decodeAsync(await result.json());
    return pet;
  };
  

• map(func: (t: T) => T2): Decoder<T2> - each decoder is a functor. Map allows you to apply a function to an underlying decoder value, provided that decoding succeeded. Map accepts a function of type (t: T) -> T2, where T is a type of decoder (and underlying value), and T2 is a type of resulting decoder.

• then(bindFunc: (t: T) => Decoder<T2>): Decoder<T2> - allows for monadic chaining of decoders. Takes a function, that returns a Decoder<T2>, and returns a Decoder<T2>

Custom decoder

Result and pattern matching

Decoding can either succeed or fail, to denote that json-decoder has ADT type Result<T>, which can take two forms:

• Ok<T> - carries a succesfull decoding result of type T, use .value to access value
• Err<T> - carries an unsuccesfull decoding result of type T, use .message to access error message

Result also has functorial map function that allows to apply a function to a value, provided that it exists

const r: Result<string> = Ok("cat").map(s => s.toUpperCase()); //Ok("CAT")
const e: Result<string> = Err("some error").map(s => s.toUpperCase()); //Err("some error")

It is possible to pattern-match (using poor man's pattern matching provided by TypeScript) to determite the type of Result

// assuming some result:Result<Person>

switch (result.type) {
  case OK: result.value; // Person
  case Err: result.message; // message string
}

Friendly errors

TBC

Mapping and type conversion

TBC

Validation

JSON only exposes an handful of types: string, number, null, boolean, array and object. There's no way to enforce special kind of validation on any of above types using just JSON. json-decoder allows to validate values against a predicate.

Example: integerDecoder - only decodes an integer and fails on a float value

const integerDecoder: Decoder<number> = numberDecoder.validate(n => Math.floor(n) === n, "not an integer");
const integer = integerDecoder.decode(123); //Ok(123)
const float = integerDecoder.decode(123.45); //Err("not an integer")

Example: emailDecoder - only decodes a string that matches email regex, fails otherwise

const emailDecoder: Decoder<number> = stringDecoder.validate(/^\S+@\S+$/.test, "not an email");
const email = emailDecoder.decode("[e-mail]"); //Ok("[e-mail]")
const notEmail = emailDecoder.decode("joe"); //Err("not an email")

Also decoder.validate can take function as a second parameter. It should have such type: (value: T) => string.

Example: emailDecoder - only decodes a string that matches email regex, fails otherwise

const emailDecoder: Decoder<number> = stringDecoder.validate(/^\S+@\S+$/.test, (invalidEmail) => `${invalidEmail} not an email`);
const email = emailDecoder.decode("[e-mail]"); //Ok("[e-mail]")
const notEmail = emailDecoder.decode("joe"); //Err("joe is not an email")

Contributions are welcome

Please raise an issue or create a PR