Declaration TypeScript Runner (dtsr): The easiest way to run your .d.ts files

Getting started    |    Community examples →

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• Getting started
  • Hello World
  • Eval a type expression in a file context
• Documentation
  • Overview
  • Usage
  • How it works
  • CLI options in detail
  • Basic example
  • Advanced usage
  • Common Gotchas
• Community Examples
  • FizzBuzz.d.ts
  • Fibonacci.d.ts
  • ParseQueryString.d.ts
• Is this project for real?

Getting started

You can install dtsr with:

npm install --global dtsr

Or run directly with npx:

npx dtsr --help

Hello World

Create a file Hello.d.ts:

export type Main<Argv extends string[]>
  = `Hello ${Argv[0]}!`

Run with:

> dtsr ./Hello.d.ts world
"Hello world!"

Tip

Making .d.ts executables:

You can make your .d.ts files executable by adding the shebang line: #!/usr/bin/env dtsr _{(don't forget to chmod +x ./Hello.d.ts)}

Eval a type expression in a file context

By default, dtsr pass the command-line arguments to the type Main on the passed file, but you can also evaluate a custom expression using the --eval flag:

> dtsr --eval 'Main<["world"]>' ./Hello.d.ts
"Hello world!"

In fact, if you just want to quickly evaluate a type expression, you don't even need to create a file:

> dtsr --eval '"world" extends string ? "yes" : "no"'
"yes"

Tip

Checkout our Community Examples to see more complex use cases.

Documentation

Overview

dtsr (Declaration TypeScript Runner) lets you evaluate types from a TypeScript declaration file (.d.ts), a normal TypeScript (.ts, .tsx) or directly from an inline expression. You can think of it as a quick way to “run” your type-level logic instead of hoping that hovering a symbol in VSCode will not truncate the type beyond recognition.

Usage

Usage: dtsr [options] <source-file> [args...]

Evaluate type-level expressions from TypeScript declaration files.

Options:
  -v, --version            Output the version number
  -e, --eval <expression>  Evaluate a type expression in the context of the source file
  -p, --project <path>     Specify the TypeScript configuration file (default: tsconfig.json)
  -h, --help               Display this help message

Examples:
  $ dtsr ./Main.d.ts "Joe"
  $ dtsr --eval 'Foo<string, number>' ./Main.d.ts
  $ dtsr --project tsconfig.prod.json ./Main.d.ts "Joe"
  $ dtsr --eval '"hello" extends string ? "yes" : "no"'

How it works

1. With a source file: By default, dtsr looks for a type named Main in your .d.ts file.

   • If you pass extra arguments ([args...]) after the file name, these will be provided as Main<[...args]>.
   • If no additional arguments are given, it simply evaluates Main.

2. With --eval: Instead of looking for Main, dtsr will create a temporary type alias to evaluate whatever expression you provide.

   • If you also specify a <source-file>, that file's types will still be in scope, so you can reference them from your expression.

3. Reading configuration: By default, dtsr tries to find and use a local tsconfig.json to respect any custom compiler settings you might have.

   • If you want to point to a specific tsconfig file, use -p or --project.

4. Behind the scenes:

   • dtsr creates a in-memory TypeScript file that inlines your expression as type __dtsr_result_type = ....
   • It then uses the TypeScript compiler APIs to compute the type's string representation.

CLI options in detail

• <source-file>
  The .d.ts file you want to evaluate. Must be provided if you do not use --eval.

• [args...]
  Additional string arguments appended after the <source-file>. These get fed into your Main type as Main<[...args]>.

• -e, --eval <expression>
  Evaluates a custom type expression:

  dtsr --eval "Foo<string, number>"

  If a <source-file> is also specified, you can reference its exported types in your expression.

• -p, --project <path>
  Specifies a custom tsconfig.json to use instead of automatically searching for one in the current directory:

  dtsr --project tsconfig.prod.json ./MyTypes.d.ts

• -v, --version
  Displays the current version of dtsr.

• -h, --help
  Displays the usage help message.

Basic example

Hello.d.ts:

export type Main<Argv extends string[]>
  = `Hello ${Argv[0]}!`

Run:

> dtsr ./Hello.d.ts world
"Hello world!"

The use of command line arguments is optional, instead you can:

export type Main = "Hello world!"

Run:

> dtsr ./Hello.d.ts
"Hello world!"

Or you can just evaluate an expression using the types defined on the file:

> dtsr --eval '`${Main} How are you?`' ./Hello.d.ts
"Hello world! How are you?"

Advanced usage

1. Evaluate any expression without a file
   You can skip specifying a source file entirely by just using --eval. For instance:

> dtsr --eval '"world" extends string ? "yes" : "no"'
"yes"

2. Use with a custom tsconfig
   If you have a custom tsconfig file, you can specify it with --project:

> dtsr --project tsconfig.prod.json ./MyTypes.d.ts

3. Make files directly executable
   Add the shebang line #!/usr/bin/env dtsr to the top of your file, and make them executable with chmod +x ./main. Now you can run them directly from the command line:

   #!/usr/bin/env dtsr
   // file: ./main
   export type Main = "Hello World!"

   Now you can run ./main directly from the terminal.

   [!NOTE] For the shebang to work, you need to have dtsr installed globally or in your PATH.

Common Gotchas

• No Main type found: If you don't provide --eval and dtsr can't find Main in the file, you'll get an error. Either rename your main entry type to Main or use --eval.

• Invalid arguments: All [args...] passed after the source file are interpreted as strings. Make sure your Main type is expecting that structure (string[]).

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For more examples, head over to the Community Examples section to see how you can leverage type-level programming to solve interesting problems.

Community Examples

FizzBuzz.d.ts

> dtsr ./FizzBuzz.d.ts 15
["1", "2", "Fizz", "4", "Buzz", "Fizz", "7", "8", "Fizz", "Buzz", "11", "Fizz", "13", "14", "FizzBuzz"]

type Main<Argv extends string[]>
  = Take<ParseInt<Argv[0]>, FizzBuzzIterator[1]>

type FizzBuzzIterator<
  S extends Record<any, any> = { 3: []; 5: []; i: [] },
  R extends string = `${S[3]['length'] extends 3 ? 'Fizz': ''}${S[5]['length'] extends 5 ? 'Buzz' : ''}`,
> = [
  R extends '' ? `${S['i']['length']}` : R,
  FizzBuzzIterator<{
    3: S[3]['length'] extends 3 ? [1] : [...S[3], 1]
    5: S[5]['length'] extends 5 ? [1] : [...S[5], 1]
    i: [...S['i'], 1]
  }>,
]

type Take<N extends number, Cons extends [any, any], Acc extends any[] = []> = Acc['length'] extends N ? Acc : Take<N, Cons[1], [...Acc, Cons[0]]>
type ParseInt<S extends string> = S extends `${infer N extends number}` ? N : never

Fibonacci.d.ts

> dtsr ./Fibonacci.d.ts 10
55

type Main<Argv extends string[]>
  = Fib<ParseInt<Argv[0]>>

type Fib<X extends number>
  = X extends 0 ? 0
  : X extends 1 ? 1
  : Add<Fib<Sub<X, 1>>, Fib<Sub<X, 2>>>

type ParseInt<S extends string> = S extends `${infer N extends number}` ? N : never
type Repeat<N extends number, Acc extends true[] = []> = Acc['length'] extends N ? Acc : Repeat<N, [...Acc, true]>
type Tail<T extends any[]> = T extends [any, ...infer U] ? U : never
type Add<A extends number, B extends number> = [...Repeat<A>, ...Repeat<B>]['length']
type Dec<A extends number> = Tail<Repeat<A>>['length']
type Sub<A extends number, B extends number> = B extends 0 ? A : Sub<Dec<A>, Dec<B>>

ParseQueryString.d.ts

> dtsr ./ParseQueryString.d.ts 'xs=1&xs=2&y=3&flag'
{ xs: ["1", "2"]; flag: [""]; y: ["3"]; }

type Main<Argv extends string[]>
  = Parse<Argv[0]>

type Parse<Q extends string>
  = Q extends '' ? {}
  : Q extends `${infer Head}&${infer Tail}` ?
    ([Parse<Head>, Parse<Tail>] extends [infer A, infer B] ?
      { [K in keyof A | keyof B]: [...(K extends keyof A ? Extract<A[K], any[]> : []), ...(K extends keyof B ? Extract<B[K], any[]> : []) ] } : never)
  : Q extends `${infer K}=${infer V}` ? { [P in K]: [V] }
  : { [P in Q]: [''] }

Is this project for real?

No _{(But it does work as described)}