Summary - Assertion functions
- TypeScript course
- Advanced
- Lesson 44/57
- 4 min
Write a safe request handler with assertion functions.
Summary - Assertion functions
Assertion functions form a safe validation layer in a request handler. asserts x is defined checks for null, asserts x is T checks the type. Multiple assertion calls guarantee the request meets all requirements; the subsequent code runs safely.
function assertDefined(x: unknown): asserts x is defined {
if (x === null || x === undefined) throw new Error("missing");
}
function assertString(x: unknown): asserts x is string {
if (typeof x !== "string") throw new Error("not string");
}
function handle(input: unknown): string {
assertDefined(input);
assertString(input);
return input.toUpperCase();
}Practice Write a request handler
Define an assertDefined(x: unknown): asserts x is defined function (with throw new Error("missing") when x === null || x === undefined). Define an assertString(x: unknown): asserts x is string function (with throw new Error("not string") when typeof x !== "string"). Write a handle(input: unknown): string function: call assertDefined(input), call assertString(input), then return input.toUpperCase(). Print the result via console.log(handle("hello")).
Starter code
// assertDefined fonksiyonunu tanımla (asserts x is defined, throw ile)
// assertString fonksiyonunu tanımla (asserts x is string, throw ile)
// handle(input: unknown): string fonksiyonunu iki assertion ile yaz
// handle("hello") sonucunu yazdırSample solution
function assertDefined(x: unknown): asserts x is defined {
if (x === null || x === undefined) {
throw new Error("missing");
}
}
function assertString(x: unknown): asserts x is string {
if (typeof x !== "string") {
throw new Error("not string");
}
}
function handle(input: unknown): string {
assertDefined(input);
assertString(input);
return input.toUpperCase();
}
console.log(handle("hello"));Expected output
HELLO
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why are multiple assertion calls made in a request handler?
- A single assertion is enough
- A separate assertion per validation step keeps the code safe and organized Correct answer
- Assertions are skipped at runtime
- It increases runtime speed
Explanation: Making a separate assertion call for each validation step (null check, type check, etc.) modularizes the validation; each step has a single responsibility and the code stays safe.
2 Assertion chain in handle function
function handle(input: unknown): string {
assertDefined(input);
____
return input.toUpperCase();
}function handle(input: unknown): string {
assertDefined(input);
assertString(input);
return input.toUpperCase();
}Explanation: After the null check with assertDefined, assertString(input) performs the type check; on the next line input is safely used as string.
3 Assertion functions summary matching
| Term | Meaning |
|---|---|
asserts x is defined |
narrows out null and undefined |
asserts x is T |
narrows to T |
throw yapma |
when condition fails |
Assertion zinciri |
multiple validations |