Typescript snippet example

discriminating-union.ts

// Select matched object

// Bad implementation
// type ApiResponse {
//     state: 'success' | 'failed'
//     data?: { name: string }
//     error?: { message: string }
// }

type ApiResponseSuccess = {
    state: 'success'
    data: { name: string }
}

type ApiResponseFailed = {
    state: 'failed'
    error: { message: string }
}

type ApiResponse = ApiResponseSuccess | ApiResponseFailed

const processResponse = (response: ApiResponse) => {
    if (response.state === 'success') {
        console.log(response.data.name)
    }

    if (response.state === 'failed') {
        console.log(response.error.message)
    }
}

type-guard.ts

// Narrowing down type

type Person = {
    type: 'person'
    name: string
    age: number
}

type Company = {
    type: 'company'
    name: string
    employeeCount: number
}

type Entity = Company | Person

// Bad implementation
// function isPerson(entity: Entity) {
//     return entity.type === 'person'
// }

function isPerson(entity: Entity): entity is Person {
    return entity.type === 'person'
}

function checkEntity(entity: Entity) {
    if (isPerson(entity)) {
        console.log(entity.age)
    }
}