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TypeScript Narrowing and Type Guards Cheat Sheet

A 2-page reference for narrowing and type guards: 14 short entries with copy-ready code.

ForDevelopers

typescript-narrowing-and-type-guards-cheat-sheet.pdfShowing 21 of 81 lines
// typeof guard
function f(x: string | number) {
  if (typeof x === "string") return x.toUpperCase();
  return x.toFixed(2);
}

// Truthiness narrowing
function len(s?: string) {
  return s ? s.length : 0;
}

// Equality narrowing
function g(x: "a" | "b") {
  if (x === "a") {
    // x is "a"
  }
}

// in operator
type Cat = { meow(): void };
type Dog = { bark(): void };

What's inside

Format
PDF, Markdown
Contents
14 sections · 2 pages
Version
1.0 · Updated 3 Oct 2026
Licence
Commercial What this means
Checked
Checked before it ships
  • typescript-narrowing-and-type-guards-cheat-sheet.pdfPDF · 21 KB
  • typescript-narrowing-and-type-guards-cheat-sheet.mdMarkdown · 2 KB
  • LICENSE.txtadded to every download

About this item

A compact, printable reference for narrowing and type guards. Each of the 14 entries has a one-line explanation and a small example you can paste into your editor. The PDF uses a two-column A4 layout designed to sit next to your keyboard, and the same content ships as Markdown so you can search it or drop it into your notes. Every TypeScript example targets TypeScript 5.

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