TypeScript中的怪语法

{
    "compilerOptions": {
        "strict": true,
        //...
    }
}

var name: string;   // cannot be null and undefined.

name = undefined;    // Error: [ts] Type 'undefined' is not assignable to type 'string'.
name = null;         // Error: [ts] Type 'null' is not assignable to type 'string'.
console.log(name);   // Error: [ts] Variable 'address' is used before being assigned.

var address: string | undefined; // can be undefined

class Person {
    name: string;               // cannot be null and undefined
    address?: string;           // can be undefined
}

var person : Person = {name: "Joe"};

console.log(person.address.toString()); // Error: [ts] Object is possibly 'undefined'.

if (person.address != undefined) {
    console.log(person.address.toString()); //Ok. as we checked the type
}

class Person {
    id: number;
    name: string;
    birthday: Date;
}

type personPropKeys = keyof Person; // same as: type personPropKeys = "id" | "name" | "birthday"

var propKey : personPropKeys;
propKey = "id";     // OK
propKey = "name";   // OK
propKey = "age";    // Error: [ts] Type '"age"' is not assignable to type '"id" | "name" | "birthday"'.

// Keep types the same, but make each property to be read-only.
type Readonly<T> = {
    readonly [P in keyof T]: T[P];
};

// Same property names, but make the value a promise instead of a concrete one
type Deferred<T> = {
    [P in keyof T]: Promise<T[P]>;
};

// Wrap proxies around properties of T
type Proxify<T> = {
    [P in keyof T]: { get(): T[P]; set(v: T[P]): void }
};

class Person {
    // same as to define instance fields: id, name, age
    constructor(private id: number, public name: string, readonly age: number) {
    }

    get Id() : number {
        return this.id;
    }
}

var person = new Person(1, "Mary", 14);
console.log(person.name);

// This is a generic method to create an object
function createObject<T>(name:string, creator: (arg: string) => T) : T {
    return creator(name);
}

// now we have a class Person, we want to create it via function createObject
class Person {
    public constructor(name: string) {
        this.name = name;
    }

    name: string;
}

// we have to define a creator function
function createPerson(name: string): Person {
    return new Person(name);
}

// at end, we can create a person
var person = createObject<Person>("Kate", createPerson);

// This is a generic method to create an object
function createObject<T>(name:string) : T {
    return new T(name);     // Error: [ts] 'T' only refers to a type, but is being used as a value here.
}

// now we have a class Person, we want to create it via function createObject
class Person {
    public constructor(name: string) {
        this.name = name;
    }

    name: string;
}

// at end, we can create a person
var person = createObject<Person>("Kate");

name: string;

type NewObject<T> = {new(name: string): T};     // type NewPersonType = new (name: string) => Person
var newPersonType: NewObject<Person> = Person;
var person2 = new newPersonType("Joe");

// we also can write like this, as {} is the root class of object type.
type ObjectEmpty = {new(): {}};     // type ObjectEmpty = new () => {}

function restFunction(first: string, second: string, ...args: string[]): void {
    console.log(args);      // [ 'three', 'four' ]
}

restFunction("one", "two", "three", "four");

// shadow copy
var objCopy: any = {...obj};
console.log(objCopy);       // { x: 1, y: 'name', z: 2 }
console.log(objCopy === obj);   // false

// copy and change
var obj2 = {a: "age"};
objCopy = {...obj, z: "zoo"};
console.log(objCopy);       // { x: 1, y: 'name', z: 'zoo' }

// merge
var obj2 = {a: "age"};
objCopy = {...obj, ...obj2};
console.log(objCopy);       // { x: 1, y: 'name', z: 2, a: 'age' }

// copy and remove
let {z, ...objCopy2} = obj
console.log(objCopy2);      // { x: 1, y: 'name' }