Parse JSON as Typed Instances in TypeScript

Installation

npm install ts-parse

Prereqs

• Enable emitDecoratorMetadata and experimentalDecorators in tsconfig.json:

  {
    "compilerOptions": {
      "emitDecoratorMetadata": true,
      "experimentalDecorators": true
    }
  }

Approach

Typed parsing utilizes constructor-based injection to create actual class instances from JSON or plain JavaScript objects. The Reflect Metadata shim, which is currently a ECMAScript proposal, exposes the type information of a constructor's parameters making it possible to instantiate objects from their real constructors.

For example:

@Serializable
class Address {

  constructor(private city: string, private state: string, private zip: string) {
    
  }
  
  getCityStateZip(): string {
    return `${this.city}, ${this.state} ${this.zip}`;
  }
}

let address = TSON.parse('{"city":"Richmond","state":"VA","zip":"23230"}', Address);

address instanceof Address; // true
address.getCityStateZip(); // "Richmond, VA 23230"

Features and Gotchas

• Serialize JSON or POJOs to actual class instances
• Instantiation through construction injection
• Fully supports inheritance through super constructors
• Missing arguments in the constructor will be passed as NULL
• Extra JSON properties are set to the class instance via Object.assign

Array Support

Array support is built in but requires a cast for compile-time type support. This is required to support a simplified API without creating extra parsing methods to support types.

@Serializable
class Foo { }

const json = "[{},{}]";

// extra cast is required and can't be inferred
let foos = <Foo[]> TSON.parse(json, Foo);
foos[0] instanceof Foo; // true

@Serializable
class Bar { }

const json = "[[{},{}]]";

// there's no limit to the amount of arrays that can be nested
let bars = <Bar[][]> TSON.parse(json, Bar);
bars[0][0] instanceof Bar; // true

Getters and Setters

// -------------------------------------------------------------------------------------------------
// TODO make easy getter/setter injection. 
// for example:
//
// class A {
//
//   constructor(private _foo: string} {
// 
//   }
//  
//   get foo() { return this._foo; };
//
// }
//
// then TSON.parse('{"foo":"something"}'
// will serialize into either "_foo" param or "foo" param.  if both are detected then ignore "_foo"?
// this should be an optional config item probably
// -------------------------------------------------------------------------------------------------

Configuration

• TODO Generic serialization when type information does not exist (defaults to true)
• TODO Serialize Private Arguments via prefix (defaults to "_")
• TODO Serialize extra JSON properties to class instance (defaults to true)

Alternative Approach

Another approach is to creating instances would be member injection which this project does not support.

Tradeoffs and difficulties to field assignment:

• Annotate every member (the "code bloat" is a net zero tradeoff to the constructor "bloat")
• No support for inheritance - why? because we can't clearly define what properties to exclude getOwnProperties
• Assign to -> fields and setters
• How to handle -> readonly, private, protected, const

Future Potential / Plugins

• TODO Angular4 Serialization Plugin
  • HttpClient.get(...);
  • The problem is the generic type is erased. We'd have to pass the type - integrating into the existing API would be difficult or impossible. Unless the HttpInterceptor could ... do something cool.

Contributing

• Fork and/or clone repo.
• Then npm install.
• Run a quick test npm run test.
• Make changes and run linter npm run lint.
• Submit a Pull Request!

You may need to install globals:

• npm install mocha chai ts-node typings -g
• typings install dt~mocha --global --save
• typings install npm~chai --save

Known Issues

Ordering - Dependencies must come before use or they'll be undefined.
In webpack, this might not be avoidable.
What's the workaround? See: microsoft/TypeScript#4114

One workaround would be to use getters for ElementTypes - that's a last resort. Could probably use a UnionType