Typescript：声明空类对象

Question

export class Month {
 January: string;
 February: string;
 March: string;
 ...
}

mapMyPayload(specialMonths: Birthdays) {
 let myMonth = new Month;
 console.log(myMonth); //=> Month {}
    // here I expect the Object.keys(myMonth) 
    // returns {January=undefined, February=undefined, ...}
    // since I assume the new Month 
    // declares the myMonth as an object with undefined as its value
 Object.keys(myMonth).forEach(m => {
    // iterate thru the keys 
    // check if the month exists inside the specialMonths
    // and is not null or undefined
  if (specialMonths[m] != null) { 
   myMonth[m] = specialMonths[m];
 );
 return myMonth;
}

我查看了许多示例代码和文档，无论是隐式构造函数还是显式构造函数，您都可以通过使用类名前面的new来声明新类，但随后它们会声明一些值。因此，我认为在某个外部作用域声明它之前，实例是不存在的。

Answer 1

class Foo {
  a: string;
  b: string;
  c: number;
}

var foo = new Foo();
console.log(foo.a); //undefined

更新：这将转换为以下JS代码：

var Foo = /** @class */ (function () {
    function Foo() {
    }
    return Foo;
}());
var foo = new Foo();
console.log(foo.a);

您的对象没有任何键，因为它们还没有定义，所以它们求助于“undefined”文字而不是“undefined”作为值。您可以尝试将“undefined”默认为值：

class Foo {
  a: string = undefined;
  b: string = undefined;
  c: number = undefined;
}

var foo = new Foo();
console.log(foo.a); //undefined
console.log(foo); //=> Foo{a: undefined, b: undefined, c: undefined}
Object.keys(foo).forEach(property => console.log(property));
// keys log as empty and not undefined

这将传递到以下JS：

var Foo = /** @class */ (function () {
    function Foo() {
        this.a = undefined;
        this.b = undefined;
        this.c = undefined;
    }
    return Foo;
}());
var foo = new Foo();
console.log(foo.a); //undefined
console.log(foo); //=> Foo{a: undefined, b: undefined, c: undefined}
Object.keys(foo).forEach(function (property) { return console.log(property); });
// keys log as empty and not undefined as part of the forEach loop