Summary - Assertion functions

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();
}

Exercise 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")).

Show example 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"));

Test yourself

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
- Assertions are skipped at runtime
- It increases runtime speed
Show answer

A separate assertion per validation step keeps the code safe and organized — 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.

Assertion chain in handle function

function handle(input: unknown): string {
  assertDefined(input);
  ____
  return input.toUpperCase();
}

Show answer

function handle(input: unknown): string {
  assertDefined(input);
  assertString(input);
  return input.toUpperCase();
}

Assertion functions summary matching

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

Next lesson: strictNullChecks deep

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