Narrowing means checking a value so TypeScript knows its exact type in a branch:
typeof x === "string"x instanceof Date"swim" in animal- Discriminated unions: a shared
kindproperty tells the variants apart.
TypeScript Narrowing & Type Guards
Help TypeScript understand which type you have.
✍️ 3 exercises + code challenge in this lessonKey Takeaways: TypeScript Narrowing & Type Guards
Mastering TypeScript Narrowing & Type Guards helps you write clean and reliable TypeScript code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.
TypeScript Narrowing & Type Guards Code Example
TypeScripttype Circle = { kind: "circle"; radius: number };
type Square = { kind: "square"; side: number };
type Shape = Circle | Square;
function area(s: Shape): number {
switch (s.kind) {
case "circle":
return Math.round(Math.PI * s.radius ** 2);
case "square":
return s.side ** 2;
}
}
console.log(area({ kind: "circle", radius: 2 }), area({ kind: "square", side: 3 }));Output
13 9
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TypeScript Narrowing & Type Guards Practice Exercises
Check what you learned in “TypeScript Narrowing & Type Guards”.
/ 3 solved
Fill in the blank
Check whether value is a Date.
if (value Date) { }
✓ Correct!
Not quite. Try again.
Multiple choice
What is a discriminated union?
✓ Correct!
Not quite. Try again.
Code challenge
Write len(x: string | string[]) that returns x.length for both. Log len("abc") and len(["a", "b"]) separated by a space.
Write the code in the live editor. Your answer is checked automatically.
✓ Solved
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