Prettier Plugin Explicit Return

A Prettier plugin that automatically adds explicit return type annotations to TypeScript functions using type inference.

Installation

npm install --save-dev prettier-plugin-explicit-return

Usage

Add the plugin to your Prettier configuration:

.prettierrc.json:

{
  "plugins": ["prettier-plugin-explicit-return"]
}

package.json:

{
  "prettier": {
    "plugins": ["prettier-plugin-explicit-return"]
  }
}

Then format your TypeScript files as usual:

npx prettier --write "**/*.{ts,tsx}"

Features

This plugin automatically adds explicit return type annotations to:

• ✅ Function declarations
• ✅ Arrow functions
• ✅ Class methods
• ✅ Function expressions
• ✅ Async functions (including Promise return types)

Functions that already have explicit return types are skipped to avoid duplication.

Examples

Function Declaration

Before:

function sum(a: number, b: number) {
  return a + b;
}

After:

function sum(a: number, b: number): number {
  return a + b;
}

Arrow Function

Before:

const multiply = (a: number, b: number) => {
  return a * b;
};

After:

const multiply = (a: number, b: number): number => {
  return a * b;
};

Class Method

Before:

class Calculator {
  add(a: number, b: number) {
    return a + b;
  }
}

After:

class Calculator {
  add(a: number, b: number): number {
    return a + b;
  }
}

Function Expression

Before:

const handler = function (event: string) {
  return event.length;
};

After:

const handler = function (event: string): number {
  return event.length;
};

Async Function

Before:

async function fetchData(url: string) {
  return fetch(url);
}

After:

async function fetchData(url: string): Promise<Response> {
  return fetch(url);
}

Existing Return Types (Skipped)

Functions with existing return types are left unchanged:

Before & After:

function testVoid(): void {
  console.log("testVoid");
}

How It Works

The plugin uses TypeScript's type checker to infer return types for functions that don't have explicit return type annotations. It:

1. Parses your TypeScript code using TypeScript's compiler API
2. Uses the type checker to infer return types
3. Adds explicit return type annotations to functions that don't have them
4. Preserves all formatting and existing code structure
5. Works seamlessly with Prettier's formatting pipeline

Requirements

• Node.js 14+
• Prettier 3.6+
• TypeScript 5.9+

License

MIT © 2025 Gavin Harris