Exhaustiveness Checking

Use Never Types for Exhaustiveness Checking

Overview

Use never to ensure all cases in a union are handled.

When you add a new variant to a union type, TypeScript can automatically flag every switch statement that needs updating. This catches errors of omission at compile time.

When to Use This Skill

• Handling all cases of a tagged union
• Adding new variants to discriminated unions
• Writing switch statements that must be complete
• Want compile-time errors when cases are missed

The Iron Rule

ALWAYS add exhaustiveness checking to switch statements on union types.

Remember:

• After exhaustive cases, the type is never
• Nothing is assignable to never except never
• Missing cases turn into type errors
• This catches errors of omission

Detection: The Missing Case Problem

Without exhaustiveness checking, new union variants are silently ignored:

type Shape = Box | Circle | Line;

function drawShape(shape: Shape, ctx: CanvasRenderingContext2D) {
  switch (shape.type) {
    case 'box':
      ctx.rect(...shape.topLeft, ...shape.size);
      break;
    case 'circle':
      ctx.arc(...shape.center, shape.radius, 0, 2 * Math.PI);
      break;
    // Forgot 'line' - NO ERROR! Lines silently don't draw.
  }
}

The Exhaustiveness Pattern

The assertUnreachable Helper

function assertUnreachable(value: never): never {
  throw new Error(`Unexpected value: ${value}`);
}

Using It in Switch Statements

function drawShape(shape: Shape, ctx: CanvasRenderingContext2D) {
  switch (shape.type) {
    case 'box':
      ctx.rect(...shape.topLeft, ...shape.size);
      break;
    case 'circle':
      ctx.arc(...shape.center, shape.radius, 0, 2 * Math.PI);
      break;
    default:
      assertUnreachable(shape);
      // If we missed a case, shape won't be 'never' and we get a type error!
  }
}

When You Add a New Type

// Add a new shape type
interface Line {
  type: 'line';
  start: Coord;
  end: Coord;
}
type Shape = Box | Circle | Line;  // Added Line

// Now drawShape shows an error:
function drawShape(shape: Shape, ctx: CanvasRenderingContext2D) {
  switch (shape.type) {
    case 'box': /* ... */ break;
    case 'circle': /* ... */ break;
    default:
      assertUnreachable(shape);
      //                ~~~~~ 
      // Argument of type 'Line' is not assignable to parameter of type 'never'
  }
}

Fix by handling the new case:

function drawShape(shape: Shape, ctx: CanvasRenderingContext2D) {
  switch (shape.type) {
    case 'box':
      ctx.rect(...shape.topLeft, ...shape.size);
      break;
    case 'circle':
      ctx.arc(...shape.center, shape.radius, 0, 2 * Math.PI);
      break;
    case 'line':
      ctx.moveTo(...shape.start);
      ctx.lineTo(...shape.end);
      break;
    default:
      assertUnreachable(shape);  // Now shape is 'never', no error!
  }
}

How It Works

After handling all cases, the remaining type is never:

function processShape(shape: Shape) {
  switch (shape.type) {
    case 'box': break;
    case 'circle': break;
    case 'line': break;
    default:
      shape
      // ^? (parameter) shape: never
  }
}

If you miss a case, the type isn't never:

function processShape(shape: Shape) {
  switch (shape.type) {
    case 'box': break;
    case 'circle': break;
    // (forgot 'line')
    default:
      shape
      // ^? (parameter) shape: Line
  }
}

Since Line is not assignable to never, you get a type error.

Alternative: Return Type Enforcement

You can also use return types to enforce exhaustiveness:

function getShapeName(shape: Shape): string {
  switch (shape.type) {
    case 'box':
      return 'Box';
    case 'circle':
      return 'Circle';
    // Missing 'line' - TypeScript error!
    // Function lacks ending return statement and return type does not include 'undefined'
  }
}

This only works if:

• The function has an explicit return type
• All cases must return
• strictNullChecks is enabled

Complete Example

// Types
type Coord = [x: number, y: number];

interface Box {
  type: 'box';
  topLeft: Coord;
  size: Coord;
}

interface Circle {
  type: 'circle';
  center: Coord;
  radius: number;
}

interface Line {
  type: 'line';
  start: Coord;
  end: Coord;
}

type Shape = Box | Circle | Line;

// Helper
function assertUnreachable(value: never): never {
  throw new Error(`Unexpected value: ${value}`);
}

// Usage - guaranteed to handle all shapes
function getArea(shape: Shape): number {
  switch (shape.type) {
    case 'box':
      return shape.size[0] * shape.size[1];
    case 'circle':
      return Math.PI * shape.radius ** 2;
    case 'line':
      return 0;  // Lines have no area
    default:
      return assertUnreachable(shape);
  }
}

When NOT to Use

Sometimes you intentionally want to ignore some cases:

function handleCommonShapes(shape: Shape) {
  switch (shape.type) {
    case 'box':
    case 'circle':
      // Handle common cases
      break;
    // Intentionally ignore 'line' - don't add assertUnreachable here
  }
}

Pressure Resistance Protocol

1. "The Default Case Handles It"

Pressure: "We have a default case, so it's fine"

Response: A silent default hides bugs when new variants are added.

Action: Use assertUnreachable in default to make missing cases explicit.

2. "We'll Remember to Update"

Pressure: "We know where to add new cases"

Response: Human memory fails. Compiler checking doesn't.

Action: Let TypeScript track it for you.

Red Flags - STOP and Reconsider

• Switch on union type without exhaustiveness check
• Default cases that silently do nothing
• Adding variants to unions without checking all usages
• "TODO: handle new case" comments

Common Rationalizations (All Invalid)

Quick Reference

// The pattern
function assertUnreachable(value: never): never {
  throw new Error(`Unexpected value: ${value}`);
}

// In switch statements
switch (union.type) {
  case 'a': /* ... */ break;
  case 'b': /* ... */ break;
  default:
    assertUnreachable(union);  // Type error if cases missing
}

The Bottom Line

Turn missing cases into compile-time errors with never.

When handling tagged unions, add assertUnreachable(value) to your default case. This ensures that adding new variants to the union produces type errors everywhere the union is handled, catching errors of omission at compile time rather than runtime.

Reference

Based on "Effective TypeScript" by Dan Vanderkam, Item 59: Use Never Types to Perform Exhaustiveness Checking.