Summary - Type guards
- TypeScript course
- Advanced
- Lesson 40/57
- 4 min
Write a safe data validation layer with type guards.
Summary - Type guards
Type guards can be combined to form a safe data validation layer. A user-defined guard narrows with is; an assertion function narrows by throwing with asserts. Primitive guards check primitive types with typeof. Together they safely process an unknown input.
function isString(x: unknown): x is string {
return typeof x === "string";
}
function assertNumber(x: unknown): asserts x is number {
if (typeof x !== "number") throw new Error("not number");
}
function process(v: unknown): string {
if (isString(v)) return `string: ${v}`;
assertNumber(v);
return `number: ${v}`;
}Practice Write a data validation layer
Define an isString(x: unknown): x is string guard function (with typeof x === "string"). Define an assertNumber(x: unknown): asserts x is number function (with throw new Error("not number") when typeof x !== "number"). Write a process(v: unknown): string function: if isString(v) is true, return `string: ${v}`; otherwise call assertNumber(v) and return `number: ${v}`. Print results via console.log(process("hello")) and console.log(process(42)).
Starter code
// isString guard fonksiyonunu tanımla (string predicate, typeof ile)
// assertNumber fonksiyonunu tanımla (asserts x is number, throw ile)
// process(v: unknown): string fonksiyonunu guard ve assert ile yaz
// process("hello") ve process(42) sonuçlarını yazdırSample solution
function isString(x: unknown): x is string {
return typeof x === "string";
}
function assertNumber(x: unknown): asserts x is number {
if (typeof x !== "number") {
throw new Error("not number");
}
}
function process(v: unknown): string {
if (isString(v)) {
return `string: ${v}`;
}
assertNumber(v);
return `number: ${v}`;
}
console.log(process("hello"));
console.log(process(42));Expected output
string: hello number: 42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the core difference between a type guard (is) and an assertion function (asserts)?
- Guards work only with classes
- A guard returns true/false, an assertion throws or narrows Correct answer
- Assertions work only with primitive types
- They are the same
Explanation: A type guard with is returns true/false and narrows in an if; an assertion function with asserts either completes to narrow or throws, with no if needed.
2 assertNumber assertion
function assertNumber(x: unknown): ____ {
if (typeof x !== "number") throw new Error("not number");
}function assertNumber(x: unknown): asserts x is number {
if (typeof x !== "number") throw new Error("not number");
}Explanation: The asserts x is number return type tells TypeScript that x is a number when the function completes; if it throws, no narrowing happens.
3 Type guards summary matching
| Term | Meaning |
|---|---|
is (type guard) |
returns true/false |
asserts (assertion) |
throw or narrow |
typeof |
primitive type check |
unknown |
safe narrowing with guard |