const numbers: Array<number> = []; // Error: Type "Array<number>" is incompatible with type "Array<number | string>" const numbersOrStrings: Array<string | number> = numbers; // Error: Property "toLocaleString" does not exist in "Number | undefined" const localeString = numbers[1].toLocaleString();

function provideNumber(providedData: ?number) { // Error: Type "number | undefined" is incompatible with type "boolean" if (!providedData) { // Oops you lost "0" throw new TypeError("Number was not provided"); } return providedData; }

// Inferenced as "<_q, _c>((_c) => _q) => (_c) => Promise<_q>" const promisify = fn => arg => Promise.resolve(fn(arg)); // Inferenced as "<_c>(_c) => Promise<_c>" const id = promisify(x => x); // Inferenced as "Promise<string>" const upperStr = id("It will be inferenced").then(str => str.toUpperCase()); // Inferenced as "Promise<number>" const twicedNum = id(42).then(num => num ** 2);

// Inferenced type "(unknown) => undefined throws TypeError" function assertNumber(arg) { if (typeof arg !== "number") { throw new TypeError("Given arg is not a number"); } } // Inferenced type "(unknown) => undefined throws ReferenceError | TypeError" function assertField(obj) { if (typeof obj === "object" && obj !== null && "value" in obj) { assertNumber(obj.value) } throw new ReferenceError('"value" property doesn\'t exist'); } try { assertField({}); } catch(error) { // error type is "ReferenceError | TypeError | unknown" }