Expand My Type

In TypeScript, expanding a type (also known as computing, resolving, simplifying, unpacking, or unwrapping) refers to converting a type expression into a flattened type that doesn't reference other types. This is mostly done to improve the readability of type hints shown in editors/IDEs.

Expand My Type provides a programmatic way to expand type expressions in TypeScript (see How It Works section for more information). It gets the source code as input, along with a type expression and returns the expanded form as a string. That can be useful for code-generation, testing, and debugging complex type errors.

Under the hood, it uses the TypeScript Compiler API to expand the type expression and optionally formats the output using Prettier.

CLI

Installation

To install the CLI globally, run:

npm install -g expand-my-type

Alternatively, the CLI can be run using npx:

npx expand-my-type

CLI Usage

Usage:
  expand-my-type [options] <source-file> <expression>

Options:
  -h, --help                    Show this help message
  -p, --prettify                Prettify the output (default)
  -P, --no-prettify             Do not prettify the output
  -c, --tsconfig <file>         Use the specified tsconfig.json file

Given a file named example.ts with the following contents:

interface SomeInterface {
  a: number;
  b?: string;
  c: number | undefined;
}

type SomeType = {
  d: number;
  e: SomeInterface;
  f: () => void;
};

Running the following command expands the type expression SomeType["e"]:

expand-my-type example.ts 'SomeType["e"]'
# Output:
# { a: number; b?: string; c: number }

API

This package exports a function named expandMyType that can be used in one of the following ways:

1. Expand the type expression in a source file:

   Given a file named example.ts with the following contents:

   interface SomeInterface {
     a: number;
     b?: string;
     c: number | undefined;
   }
   
   type SomeType = {
     d: number;
     e: SomeInterface;
     f: () => void;
   };

   Running the following code expands the type expression SomeType:

   import { expandMyType } from "expand-my-type";
   
   const expandedType = await expandMyType({
     typeExpression: "SomeType",
     sourceFileName: "./example.ts",
   });
   
   console.log(expandedType);
   /* {
     d: number
     e: { a: number; b?: string; c: number }
     f: () => void
   } */

2. Expand the type expression in a source text:

   import { expandMyType } from "expand-my-type";
   
   const expandedType = await expandMyType({
     typeExpression: "A<number>",
     sourceText: `
        type A<T> = {
          a: string;
        } & B<T>;
       
        type B<T> = {
          b: T;
        };
      `,
   });
   
   console.log(expandedType);
   /* { a: string; b: number } */

How It Works

Expand My Type uses the following utility types to expand the type expression:

// Expands a type expression
type Expand<T> = T extends (...args: infer A) => infer R
  ? (...args: Expand<A>) => Expand<R>
  : T extends Promise<infer U>
    ? Promise<ExpandTypeArgument<U>>
    : {
        [K in keyof T]: T[K] extends string ? ExpandString<T[K]> : Expand<T[K]>;
      } & {};

type ExpandTypeArgument<T> = [T & {}] extends [never]
  ? T
  : T & {} extends void
    ? T
    : Expand<T & {}>;

// Forces a union of string literal types to be expanded
type ExpandString<T extends string> = RemoveUnderscore<AppendUnderscore<T>>;
type AppendUnderscore<T extends string> = `${T}_` extends string
  ? `${T}_`
  : never;
type RemoveUnderscore<T extends string> = T extends `${infer U}_` ? U : never;

This approach comes with some limitations. Most notably, expanding a type that leads to an infinite recursion might throw an error (when prettify option is enabled), return a truncated output, or return any.