ts-transformer-typerep

A typescript custom transformer which enables a programmer to pull down type-level(compile time) information into value level(runtime).

import { typeRep, TypeKind } from 'ts-transformer-typerep';

function keys<T>(): string[] {
  const type = typeRep<T>();

  if (type.kind === TypeKind.Object) return type.properties.map(([key]) => key);
  else return [];
}

keys<{ x: 1, y: 2, z: 3 }>(); // ['x', 'y', 'z']

WARNING⚠️: This transformer is on experiment. It's not recommended to use this in product.

Installation

npm i -D ts-transformer-typerep

Check here and here to learn how to apply custom transformers.

Reference

Type TypeRep

Type representations are values that represent types. TypeRep is type of all type representation. Since typescript types can be classified in several groups, type representations are classified in groups, too.

Type	Type Representation
any	AnyRep({ kind: TypeKind.Any })
number and its subtypes	NumberRep({ kind: TypeKind.Number, literal: number })
boolean and its subtypes	BooleanRep({ kind: TypeKind.Boolean, literal: boolean })
string and its subtypes	StringRep({ kind: TypeKind.String, literal: string })
symbol	SymbolRep({ kind: TypeKind.Symbol })
bigint and its subtypes	BigIntRep({ kind: TypeKind.BigInt, literal: bigint })
null	NullRep({ kind: TypeKind.null })
undefined	UndefinedRep({ kind: TypeKind.Undefined })
object	NonPrimitiveRep(({ kind: TypeKind.NonPrimitive })
unknown	UnknownRep({ kind: TypeKind.Unknown })
never	NeverRep({ kind: TypeKind.Never })
void	VoidRep({ kind: TypeKind.Void })
Any type constructed with Union operator but not never	UnionRep({ kind: TypeKind.Union, parts: TypeRep[] })
Any type constructed with Intersection operator	IntersectionRep({ kind: TypeKind.Intersection, parts: TypeRep[] })
Any object type with call signautre	FunctionRep({ kind: TypeKind.Function, parameters: TypeRep[], returnType: TypeRep })

Type TypeKind

TypeKind is type of values for discriminating different type representations. Different type representations are distinguished by its kind field, whose value is value of TypeKind.

const type = typeRep<number>();

if (type.kind === TypeKind.Number) console.log('It\'s a number type!');
else console.log('It\'s not a number type :(');

Type	TypeKind
any	Any
number and its subtypes	Number
boolean and its subtypes	Boolean
string and its subtypes	String
symbol	Symbol
bigint and its subtypes	BigInt
null	Null
undefined	Undefined
Any object type excluding subtypes of Object	NonPrimitive
unknown	Unknown
never	Never
void	Void
Any type constructed with Union operator but not never	Union
Any type constructed with Intersection operator	Intersection
Object	Object
Any object type with call signature	Function

Function typeRep<typeToPullDown>(): TypeRep

typeRep is a function to pull type level information into value level. It returns a type representation of given type.

typeRep<10>();
/**
{
  kind: TypeKind.Number,
  literal: 10
}
**/

WARNING⚠️: This function is not fully implemented yet, please refer to following 'Type Support Table' to check which types it supports.

Type Support Table

• Available(✅): A type is fully supported
• WIP(🚧): A type is partially supported
• Todo(📝): A type is planned to be supported

Types	Current State
Primitive types	✅
Literal types	✅
never/unknown/any/void	✅
Polymorphic types	🚧(Single type variable such as T, A only)
Enums	📝
Function types	✅
Union types	✅
Intersection types	✅
Template literal types	📝
Object types	✅