michaeljscript

const data: unknown = response.body;
const maybePerson = parsePerson(data);

if (maybePerson) {
	// maybePerson is Person
	console.log('Welcome ' + maybePerson.name);
} else {
	console.log('You are not a person!');
}

michaeljscript

function checkType<T>(value: unknown, parse: (val: unknown) => T | null): value is T {
	return parse(value) !== null;
}

michaeljscript

const data: unknown = response.body;

if (checkType(data, parsePerson)) {
	// data is Person
	console.log('Welcome ' + data.name);
} else {
	console.log('You are not a person!');
}

michaeljscript

const createTypeGuard = <T>(parse: (val: unknown) => T | null) => (value: unknown): value is T => {
	return parse(value) !== null;
}

const isPerson = createTypeGuard(parsePerson);

michaeljscript

const data: unknown = response.body;

if (isPerson(data)) {
	// data is Person
	console.log('Welcome ' + data.name);
} else {
	console.log('You are not a person!');
}

michaeljscript

interface Person {
	name: string;
	age: number;
}

michaeljscript

function isPerson(value: unknown): value is Person {
	return typeof value === 'object' && value &&
		value.hasOwnProperty('name') && 
		value.hasOwnProperty('age') && 
		typeof value.name === 'string' && 
		typeof value.age === 'number';
 }

michaeljscript

interface Person {
	name: string;
	age: number;
	something: string; 
}

michaeljscript

function parsePerson(value: unknown): Person | null {
  if (
    typeof value === "object" &&
    value &&
    value.hasOwnProperty("name") &&
    value.hasOwnProperty("age") &&
    value.hasOwnProperty("something")
  ) {
    const { name, age, something } = value;
    if (

michaeljscript

declare global {
    interface Object {
        hasOwnProperty<K extends string>(key: K): this is Record<K, unknown>;
    }
}