typescript-exercises（三）

青年人的教育是国家的基石。——富兰克林

题面：

/*

Intro:

    Since we already have some of the additional
    information about our users, it's a good idea
    to output it in a nice way.

Exercise:

    Fix type errors in logPerson function.

    logPerson function should accept both User and Admin
    and should output relevant information according to
    the input: occupation for User and role for Admin.

*/

interface User {
    name: string;
    age: number;
    occupation: string;
}

interface Admin {
    name: string;
    age: number;
    role: string;
}

export type Person = User | Admin;

export const persons: Person[] = [
    {
        name: 'Max Mustermann',
        age: 25,
        occupation: 'Chimney sweep'
    },
    {
        name: 'Jane Doe',
        age: 32,
        role: 'Administrator'
    },
    {
        name: 'Kate Müller',
        age: 23,
        occupation: 'Astronaut'
    },
    {
        name: 'Bruce Willis',
        age: 64,
        role: 'World saver'
    }
];

export function logPerson(person: Person) {
    let additionalInformation: string;
    if (person.role) {
        additionalInformation = person.role;
    } else {
        additionalInformation = person.occupation;
    }
    console.log(` - ${person.name}, ${person.age}, ${additionalInformation}`);
}

persons.forEach(logPerson);

// In case you are stuck:
// https://www.typescriptlang.org/docs/handbook/2/narrowing.html#the-in-operator-narrowing

输出Error

index.ts(58,16): error TS2339: Property 'role' does not exist on type 'Person'.
  Property 'role' does not exist on type 'User'.
index.ts(59,40): error TS2339: Property 'role' does not exist on type 'Person'.
  Property 'role' does not exist on type 'User'.
index.ts(61,40): error TS2339: Property 'occupation' does not exist on type 'Person'.
  Property 'occupation' does not exist on type 'Admin'.

答案：

/*

Intro:

    Since we already have some of the additional
    information about our users, it's a good idea
    to output it in a nice way.

Exercise:

    Fix type errors in logPerson function.

    logPerson function should accept both User and Admin
    and should output relevant information according to
    the input: occupation for User and role for Admin.

*/

interface User {
    name: string;
    age: number;
    occupation: string;
}

interface Admin {
    name: string;
    age: number;
    role: string;
}

export type Person = User | Admin;

export const persons: Person[] = [
    {
        name: 'Max Mustermann',
        age: 25,
        occupation: 'Chimney sweep'
    },
    {
        name: 'Jane Doe',
        age: 32,
        role: 'Administrator'
    },
    {
        name: 'Kate Müller',
        age: 23,
        occupation: 'Astronaut'
    },
    {
        name: 'Bruce Willis',
        age: 64,
        role: 'World saver'
    }
];

export function logPerson(person: Person) {
    let additionalInformation: string;
    if ("role" in person) {
        additionalInformation = person.role;
    } else {
        additionalInformation = person.occupation;
    }
    console.log(` - ${person.name}, ${person.age}, ${additionalInformation}`);
}

persons.forEach(logPerson);

// In case you are stuck:
// https://www.typescriptlang.org/docs/handbook/2/narrowing.html#the-in-operator-narrowing

解析：

这里主要是用到了 in 的语法，注意细节如果是

if ("role" in person && person.role) {  }

会导致

index.ts(61,40): error TS2339: Property 'occupation' does not exist on type 'Person'.
  Property 'occupation' does not exist on type 'Admin'.

所以只能使用

if ("role" in person) {  }