![](/img/trans.png)
[英]How to declare a specific type of empty array inside a object using typescript?
[英]Typescript: declare empty class object
是否可以從沒有默認值的類中聲明對象?
我有一堂課,看起來像下面;
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;
}
我想要實現的是檢查對象中是否有任何undefined
或為null
並返回此類。
我查看了許多示例代碼和文檔,無論您具有隱式構造函數還是顯式構造函數,都可以使用類名的new
infront聲明新類,但之后它們聲明一些值。 因此,我認為實例在某些外部范圍聲明之前不存在。
class Foo {
a: string;
b: string;
c: number;
}
var foo = new Foo();
console.log(foo.a); //undefined
更新:轉換為以下JS代碼:
var Foo = /** @class */ (function () {
function Foo() {
}
return Foo;
}());
var foo = new Foo();
console.log(foo.a);
您的對象沒有任何鍵,因為它們尚未定義,因此它們求助於“ undefined”文字,而不是“ undefined”作為值。 您可以嘗試按照默認值“ undefined”作為代碼行:
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
這會轉換為以下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
我個人不建議這種實現,因為它易於發生意外行為,並且與打字稿帶來的JS的靜態類型版本背道而馳。
也許如果您要詳細說明要實現的目標,我們可以為您提供更好的解決方案?
為什么不可能呢?
如果不這樣指定默認值,則類中的字段和屬性將設置為默認值,對於字符串,則為null。
export class Month {
January: string = "Something";
February: string = "Something";
March: string = "Something";
...
}
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.