type IdentityFunction = (a: number) => number;
const sum: IdentityFunction = (a: number, b: number) => {
// error TS2322: Type '(a: number, b: number) => number' is not assignable to type 'IdentityFunction'.
function concatString(a: string, b: string): string {
// error TS2322: Type 'number' is not assignable to type 'string'.
// error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
// error TS2363: The right-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.