Understand Type Narrowing

Overview

Type narrowing is the process by which TypeScript refines a type from broad to more specific based on control flow.

Master narrowing to write cleaner code without type assertions, and to help TypeScript understand your logic.

When to Use This Skill

• Working with Type | null or Type | undefined
• Handling union types like string | number
• Processing discriminated unions (tagged unions)
• Getting "possibly undefined" errors
• Avoiding type assertions in conditionals

The Iron Rule

NEVER use type assertions when narrowing would work.

No exceptions:

• Not for "it's simpler"
• Not for "I checked it already"
• Not for "TypeScript doesn't understand"

Detection: The "Assertion in Conditional" Smell

If you're using as Type inside an if block, you can probably narrow instead.

// ❌ VIOLATION: Using assertion instead of narrowing
function process(value: string | null) {
  if (value !== null) {
    console.log((value as string).toUpperCase());  // Unnecessary assertion
  }
}

// ✅ CORRECT: TypeScript narrows automatically
function process(value: string | null) {
  if (value !== null) {
    console.log(value.toUpperCase());  // value is string here
    //          ^? (parameter) value: string
  }
}

Narrowing Techniques

1. Null/Undefined Checks

const el = document.getElementById('foo');
//    ^? const el: HTMLElement | null

if (el) {
  el.innerHTML = 'Hello';
  // ^? const el: HTMLElement
} else {
  el
  // ^? const el: null
}

2. typeof Guards

function padLeft(value: string, padding: string | number) {
  if (typeof padding === 'number') {
    return ' '.repeat(padding) + value;
    //                ^? (parameter) padding: number
  }
  return padding + value;
  //     ^? (parameter) padding: string
}

3. instanceof Guards

function processDate(input: Date | string) {
  if (input instanceof Date) {
    return input.toISOString();
    //     ^? (parameter) input: Date
  }
  return new Date(input).toISOString();
  //              ^? (parameter) input: string
}

4. Property Checks (in)

interface Bird { fly(): void; }
interface Fish { swim(): void; }

function move(animal: Bird | Fish) {
  if ('fly' in animal) {
    animal.fly();
    // ^? (parameter) animal: Bird
  } else {
    animal.swim();
    // ^? (parameter) animal: Fish
  }
}

5. Discriminated Unions (Tagged Unions)

interface Circle {
  kind: 'circle';
  radius: number;
}
interface Rectangle {
  kind: 'rectangle';
  width: number;
  height: number;
}
type Shape = Circle | Rectangle;

function area(shape: Shape): number {
  switch (shape.kind) {
    case 'circle':
      return Math.PI * shape.radius ** 2;
      //               ^? (parameter) shape: Circle
    case 'rectangle':
      return shape.width * shape.height;
      //     ^? (parameter) shape: Rectangle
  }
}

6. Array.isArray

function process(input: string | string[]) {
  if (Array.isArray(input)) {
    return input.join(', ');
    //     ^? (parameter) input: string[]
  }
  return input;
  //     ^? (parameter) input: string
}

7. Throw/Return Early

function processElement(el: HTMLElement | null) {
  if (!el) {
    throw new Error('Element not found');
  }
  // After the throw, el is narrowed
  el.innerHTML = 'Hello';
  // ^? (parameter) el: HTMLElement
}

User-Defined Type Guards

When built-in narrowing isn't enough:

interface Cat { meow(): void; }
interface Dog { bark(): void; }

// Type predicate: `pet is Cat`
function isCat(pet: Cat | Dog): pet is Cat {
  return 'meow' in pet;
}

function speak(pet: Cat | Dog) {
  if (isCat(pet)) {
    pet.meow();
    // ^? (parameter) pet: Cat
  } else {
    pet.bark();
    // ^? (parameter) pet: Dog
  }
}

Common Narrowing Gotchas

typeof null is "object"

function process(value: string | object | null) {
  if (typeof value === 'object') {
    value  // Still includes null!
    // ^? string | object | null -> object | null
  }
}

// Fix: Check null explicitly first
function process(value: string | object | null) {
  if (value === null) return;
  if (typeof value === 'object') {
    value  // Now just object
    // ^? (parameter) value: object
  }
}

Falsy Values

function process(x?: number | string | null) {
  if (!x) {
    x  // Includes 0, "", null, undefined!
    // ^? string | number | null | undefined
  }
}

Callbacks Don't Preserve Narrowing

function processLater(value: { name?: string }) {
  if (value.name) {
    setTimeout(() => {
      console.log(value.name.toUpperCase());
      //          ~~~~~~~~~ Object is possibly 'undefined'
    }, 100);
  }
}

// Fix: Capture the narrowed value
function processLater(value: { name?: string }) {
  if (value.name) {
    const name = value.name;  // Capture as const
    setTimeout(() => {
      console.log(name.toUpperCase());  // OK
    }, 100);
  }
}

Pressure Resistance Protocol

1. "TypeScript Doesn't Understand My Check"

Pressure: "I checked it, but TypeScript doesn't narrow"

Response: Rework your check to use a pattern TypeScript understands.

Action: Use one of the standard narrowing patterns. Create a type guard if needed.

2. "The Assertion Is Simpler"

Pressure: "I'll just use as Type instead of an if statement"

Response: Assertions don't verify at runtime. Narrowing does.

Action: Write the check. Your code will be safer.

Red Flags - STOP and Reconsider

• as Type inside an if/switch block
• Narrowing that doesn't work (check the pattern)
• typeof x === 'object' without null check
• Falsy checks on values that could be 0 or ""
• Using ! instead of proper null checks

Common Rationalizations (All Invalid)

Quick Reference

The Bottom Line

Let TypeScript narrow types through control flow. Don't bypass it with assertions.

Use standard narrowing patterns. Create type guards when needed. Capture values before callbacks. TypeScript's narrowing is powerful - learn to work with it, not around it.

Reference

Based on "Effective TypeScript" by Dan Vanderkam, Item 22: Understand Type Narrowing.