Distinct Special Values

Use a Distinct Type for Special Values

Overview

Don't use -1, 0, or "" as special values. Use null or a distinct type.

When a function can fail or have a special case, represent it with a type that TypeScript can distinguish, not an in-domain value like -1 that's just a regular number.

When to Use This Skill

• Functions that can fail (not found, error, etc.)
• Values that have a "missing" or "unknown" state
• Wrapping APIs that use sentinel values like -1
• Designing your own return types

The Iron Rule

Special cases deserve distinct types.
Use null, undefined, or tagged unions - not -1 or "".

Remember:

• -1 is just a number, indistinguishable from other numbers
• TypeScript can't protect you from special values
• null and undefined are trackable
• Explicit error states are clearer than magic numbers

Detection: The -1 Trap

function splitAround<T>(vals: readonly T[], val: T): [T[], T[]] {
  const index = vals.indexOf(val);
  return [vals.slice(0, index), vals.slice(index + 1)];
}

splitAround([1, 2, 3, 4, 5], 6);
// Expected: error or [[1,2,3,4,5], []]
// Actual: [[1,2,3,4], [1,2,3,4,5]] (!)

Why? indexOf returns -1 for "not found", but -1 is a valid array index (counts from end).

Solution: Wrap with Distinct Type

function safeIndexOf<T>(vals: readonly T[], val: T): number | null {
  const index = vals.indexOf(val);
  return index === -1 ? null : index;
}

Now TypeScript forces you to handle both cases:

function splitAround<T>(vals: readonly T[], val: T): [T[], T[]] {
  const index = safeIndexOf(vals, val);
  return [vals.slice(0, index), vals.slice(index + 1)];
  //                   ~~~~~             ~~~~~ 
  // 'index' is possibly 'null'
}

Fixed version:

function splitAround<T>(vals: readonly T[], val: T): [T[], T[]] {
  const index = safeIndexOf(vals, val);
  if (index === null) {
    return [[...vals], []];
  }
  return [vals.slice(0, index), vals.slice(index + 1)];
}

Real-World Example: Product Price

// Bad: -1 means "unknown price"
interface Product {
  title: string;
  /** Price in dollars, or -1 if price is unknown */
  priceDollars: number;
}

// Disaster waiting to happen:
function getTotal(products: Product[]) {
  return products.reduce((sum, p) => sum + p.priceDollars, 0);
  // Whoops: products with unknown price make total negative!
}

Better:

interface Product {
  title: string;
  priceDollars: number | null;
}

function getTotal(products: Product[]) {
  return products.reduce((sum, p) => {
    if (p.priceDollars === null) {
      throw new Error(`Unknown price for ${p.title}`);
    }
    return sum + p.priceDollars;
  }, 0);
}

Why strictNullChecks Matters

Using -1 as a special value is like disabling strictNullChecks:

// @strictNullChecks: false
const truck: Product = {
  title: 'Tesla Cybertruck',
  priceDollars: null,  // ok with strictNullChecks off
};

When strictNullChecks is on, TypeScript distinguishes number from number | null. Using -1 as "unknown" bypasses this safety.

When to Use Tagged Unions

If null/undefined isn't clear enough, use a tagged union:

type RequestResult<T> = 
  | { status: 'success'; data: T }
  | { status: 'error'; error: string }
  | { status: 'pending' };

function fetchUser(id: string): RequestResult<User> {
  // ...
}

const result = fetchUser('123');
if (result.status === 'success') {
  console.log(result.data.name);  // TypeScript knows data exists
}

Common Sentinel Values to Avoid

Pressure Resistance Protocol

1. "JavaScript Uses -1"

Pressure: "indexOf returns -1, I should match that pattern"

Response: JavaScript's -1 is a historical mistake. Wrap it.

Action: Create wrapper returning T | null.

2. "It's Just a Placeholder"

Pressure: "We'll never actually use that value"

Response: Someone will forget. TypeScript won't protect you.

Action: Use a distinct type that TypeScript can track.

Red Flags - STOP and Reconsider

• Magic numbers like -1, 0, or -999
• Empty strings meaning "no value"
• Comments explaining special values
• Bugs from forgetting to check for special values

Common Rationalizations (All Invalid)

Quick Reference

// DON'T: Sentinel values
function indexOf(arr, val): number { ... }  // -1 means not found
function getPrice(): number { ... }  // -1 means unknown

// DO: Distinct types
function indexOf(arr, val): number | null { ... }
function getPrice(): number | null { ... }

// DO: Tagged unions for complex states
type Result<T> = { ok: true; value: T } | { ok: false; error: string };

The Bottom Line

Special cases deserve special types.

Using -1 or "" as special values bypasses TypeScript's type system. Use null, undefined, or tagged unions to represent special cases. TypeScript will then force you to handle them correctly.

Reference

Based on "Effective TypeScript" by Dan Vanderkam, Item 36: Use a Distinct Type for Special Values.