What is type narrowing and how does the compiler do it?

Question

Accepted Answer

**Narrowing** is how TypeScript refines a broad type (like a union) to a more specific one within a code branch, based on runtime checks. The compiler tracks control flow and updates the type accordingly.

```ts
function process(x: string | number) {
  if (typeof x === "string") {
    x.toUpperCase(); // ✅ here x is narrowed to string
  } else {
    x.toFixed(2);    // ✅ here x must be number
  }
}
```

## The narrowing tools

```ts
typeof x === "string"          // typeof guard — for primitives
x instanceof Date              // instanceof — for classes
"role" in obj                  // in operator — property existence
Array.isArray(x)               // built-in guards
if (x) { ... }                  // truthiness narrows out null/undefined/0/""
if (x === null) { ... }         // equality narrowing
```

## Discriminated unions narrow cleanly

```ts
type Shape =
  | { kind: "circle"; r: number }
  | { kind: "square"; side: number };

function area(s: Shape) {
  switch (s.kind) {            // the discriminant
    case "circle": return Math.PI * s.r ** 2;  // s is the circle variant
    case "square": return s.side ** 2;          // s is the square variant
  }
}
```

Checking the shared literal field (`kind`) tells the compiler exactly which variant you have, unlocking that variant's properties.

## Custom guards extend narrowing

```ts
function isUser(x: unknown): x is User { return !!x && typeof (x as any).name === "string"; }
```

## Why it matters

Narrowing is what makes union types *usable* — it lets you safely access type-specific members only after proving the type, turning runtime checks into compile-time guarantees.

It's the daily mechanism behind handling `string | number`, `T | null`, and tagged state.