Type example

typeExample.ts

interface IObject {
    id: string
}

// long and ugly typing
const receiveDifferentTypes = (argument: string | 5 | IObject | null): number | {key: string} | undefined | (() => string) => {
    if (argument === null) {
        return undefined
    }

    if (typeof argument === 'object' && typeof argument.id === 'string') {
        return {key: argument.id}
    }

    if (argument === 5) {
        return argument * Math.random() * 10
    }

    return (): string => 'i don\'t like strings'
}

// Single variable instead of long ugly line
type Argument = (string | 5 | IObject | null)
// Single variable instead of long ugly line
type Return = (number | {key: string} | undefined | (() => string))

// nice and tidy typing
const receiveDifferentTypes2 = (argument: Argument): Return => {
    if (argument === null) {
        return undefined
    }

    if (typeof argument === 'object' && typeof argument.id === 'string') {
        return {key: argument.id}
    }

    if (argument === 5) {
        return argument * Math.random() * 10
    }

    return (): string => 'i don\'t like strings'
}

receiveDifferentTypes('string') // Good
receiveDifferentTypes(5) // Good
receiveDifferentTypes({id: 'string'}) // Good
receiveDifferentTypes(null) // Good
receiveDifferentTypes({key: 'string'}) // Error: Argument of type '{ key: string; }' is not assignable to parameter of type 'string | 5 | { id: string; } | null'

receiveDifferentTypes2('string') // Good
receiveDifferentTypes2(5) // Good
receiveDifferentTypes2({id: 'string'}) // Good
receiveDifferentTypes2(null) // Good
receiveDifferentTypes2({key: 'string'}) // Error: Argument of type '{ key: string; }' is not assignable to parameter of type 'Argument'