---
title: "TypeScript Narrowing Patterns Cheatsheet"
slug: "typescript-narrowing-patterns"
category: "cheatsheets"
tags: ["cheatsheets", "typescript", "narrowing", "type-guards", "reference", "frontend", "backend"]
status: "stable"
last_updated: 2026-10-01
summary: "TypeScript narrowing reference: typeof, instanceof, in, type predicates, discriminated unions, assertion functions, and exhaustive switches."
related: ["[[coding/typescript-narrowing]]", "[[coding/typescript]]", "[[coding/typescript-types-vs-interfaces]]", "[[coding/typescript-strict-mode]]", "[[cheatsheets/typescript-utility-types]]", "[[coding/typescript-runtime-validation]]", "[[cheatsheets/index|Cheatsheets]]"]
---

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## Overview

Control-flow analysis narrows a variable to a more specific type inside a branch; prefer it to casting. The rationale is in [[coding/typescript-narrowing]]. Snippets compile under `strict`.

## Built-in guards

Narrow with the built-in guard that matches the check, not with a cast.

| Guard | Narrows to |
| --- | --- |
| `typeof x === "string"` (`"number"`, `"boolean"`, `"bigint"`, `"symbol"`, `"undefined"`, `"function"`) | That primitive or function type. |
| `x === null`, `x == null` | `null`; `== null` covers `null` and `undefined`. |
| `if (x)` | Removes `null`, `undefined`, `0`, `""`, `false`. |
| `err instanceof TypeError` | The class instance type. |
| `"name" in obj` | The union members that have `name`. |
| `Array.isArray(x)` | An array type. |
| `a?.b` in a condition | Removes `null` and `undefined` from `a` in the true branch. |

```typescript
function show(v: string | number | null) {
  if (v === null) return;
  return typeof v === "string" ? v.length : v.toFixed(2);   // string | number narrowed
}
```

## Type predicates

A function returning `v is T` narrows its argument at the call site.

```typescript
function isCat(a: Cat | Dog): a is Cat { return "meow" in a; }

const present = [1, undefined, 2].filter((x) => x !== undefined);   // number[] (TypeScript 5.5+ infers the predicate)
```

Before 5.5, write the predicate by hand: `.filter((x): x is number => x !== undefined)`. The compiler does not check that a handwritten predicate is truthful.

## Discriminated unions

Give every variant a literal field (`type`, `kind`) and switch on it.

```typescript
type Shape =
  | { type: "circle"; radius: number }
  | { type: "rect"; width: number; height: number };

function assertNever(x: never): never { throw new Error(`Unhandled: ${JSON.stringify(x)}`); }

function area(s: Shape): number {
  switch (s.type) {
    case "circle": return Math.PI * s.radius ** 2;
    case "rect":   return s.width * s.height;
    default:       return assertNever(s);   // compile error when a variant is missing
  }
}
```

## Assertion functions

Use assertions at trust boundaries, and validate real data with a schema.

| Signature | Effect after the call |
| --- | --- |
| `function assert(cond: unknown): asserts cond` | `cond` is truthy. |
| `function assertDefined<T>(v: T \| null \| undefined): asserts v is T` | `v` is not nullish. |

```typescript
declare const user: { name: string } | null;
assertDefined(user);
user.name;          // narrowed to { name: string }
```

## Gotchas

Avoid these; each hides an unsound narrowing.

| Gotcha | Fix |
| --- | --- |
| `typeof null === "object"`. | Check `x !== null` before using an object branch. |
| A wrong handwritten predicate or assertion silently misleads the compiler. | Keep them small, and validate untrusted data with a schema ([[coding/typescript-runtime-validation]]). |
| `"k" in x` throws at runtime when `x` is a primitive or nullish. | Guard `x` first. |
| A discriminant typed `string` or `number` narrows nothing. | Use literal types. |
| `assertNever` catches missing variants only at compile time. | At runtime it is your guard against unexpected external data. |

## Related

- [[coding/typescript-narrowing]]
- [[coding/typescript]]
- [[coding/typescript-types-vs-interfaces]]
- [[coding/typescript-strict-mode]]
- [[cheatsheets/typescript-utility-types]]