typomath/src/steps/three.ts

import overload from '../util/overload.js'

const adder = overload([
    (x: number, y: number) => {
        if (typeof x === 'number' && typeof y === 'number') return x + y
        throw new TypeError('Can only add numbers')
    },
    (x: string, y: string) => 'Yay' + x + y
] as const)

console.log(adder(1, 2))
console.log(adder('a', 'b'))
//@ts-expect-error
console.log(adder(3n, 4n))