如何在TypeScript中定义可选为空的类型？-DeclaringTypesasOptionallyNullableinTypeScript

155  

All fields in Javascript (and in TypeScript) can have the value null or undefined.

Javascript（和TypeScript）中的所有字段都可以具有null或undefined值。

You can make the field optional which is different from nullable.

您可以将字段设为可选字段，该字段与可空字段不同。

interface Employee1 {
    name: string;
    salary: number;
}

var a: Employee1 = { name: 'Bob', salary: 40000 }; // OK
var b: Employee1 = { name: 'Bob' }; // Not OK, you must have 'salary'
var c: Employee1 = { name: 'Bob', salary: undefined }; // OK
var d: Employee1 = { name: null, salary: undefined }; // OK

// OK
class SomeEmployeeA implements Employee1 {
    public name = 'Bob';
    public salary = 40000;
}

// Not OK: Must have 'salary'
class SomeEmployeeB implements Employee1 {
    public name: string;
}

Compare with:

与之比较：

interface Employee2 {
    name: string;
    salary?: number;
}

var a: Employee2 = { name: 'Bob', salary: 40000 }; // OK
var b: Employee2 = { name: 'Bob' }; // OK
var c: Employee2 = { name: 'Bob', salary: undefined }; // OK
var d: Employee2 = { name: null, salary: 'bob' }; // Not OK, salary must be a number

// OK, but doesn't make too much sense
class SomeEmployeeA implements Employee2 {
    public name = 'Bob';
}

39  

Union type is in my mind best option in this case:

在这种情况下，联盟类型是我心目中的最佳选择：

interface Employee{
   id: number;
   name: string;
   salary: number | null;
}

// Both cases are valid
let employe1: Employee = { id: 1, name: 'John', salary: 100 };
let employe2: Employee = { id: 1, name: 'John', salary: null };

28  

Just add a question mark ? to the optional field.

只需添加问号？到可选字段。

interface Employee{
   id: number;
   name: string;
   salary?: number;
}

4  

i had this same question a while back.. all types in ts are nullable, because void is a subtype of all types (unlike, for example, scala).

我有一段时间也有同样的问题.. ts中的所有类型都可以为空，因为void是所有类型的子类型（例如，scala不同）。

see if this flowchart helps - https://github.com/bcherny/language-types-comparison#typescript

看看这个流程图是否有帮助 - https://github.com/bcherny/language-types-comparison#typescript