Summary - Type guards

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}`;
}

Exercise 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)).

Show example 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));

Test yourself

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
- Assertions work only with primitive types
- They are the same
Show answer

A guard returns true/false, an assertion throws or narrows — 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.

assertNumber assertion

function assertNumber(x: unknown): ____ {
  if (typeof x !== "number") throw new Error("not number");
}

Show answer

function assertNumber(x: unknown): asserts x is number {
  if (typeof x !== "number") throw new Error("not number");
}

Type guards summary matching

is (type guard)

returns true/false
asserts (assertion)

throw or narrow
typeof

primitive type check
unknown

safe narrowing with guard

Next lesson: asserts is

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