你可以像下面这样实现一个用户定义的类型:

type Nullable<T> = T | undefined | null;

var foo: Nullable<number> = 10; // ok
var bar: Nullable<number> = true; // type 'true' is not assignable to type 'Nullable<number>'
var baz: Nullable<number> = null; // ok

var arr1: Nullable<Array<number>> = [1,2]; // ok
var obj: Nullable<Object> = {}; // ok

 // Type 'number[]' is not assignable to type 'string[]'. 
 // Type 'number' is not assignable to type 'string'
var arr2: Nullable<Array<string>> = [1,2];

你可以像下面这样实现一个用户定义的类型:

type Nullable<T> = T | undefined | null;

var foo: Nullable<number> = 10; // ok
var bar: Nullable<number> = true; // type 'true' is not assignable to type 'Nullable<number>'
var baz: Nullable<number> = null; // ok

var arr1: Nullable<Array<number>> = [1,2]; // ok
var obj: Nullable<Object> = {}; // ok

 // Type 'number[]' is not assignable to type 'string[]'. 
 // Type 'number' is not assignable to type 'string'
var arr2: Nullable<Array<string>> = [1,2];

就加个问号?到可选字段。

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

JavaScript(和TypeScript)中的所有字段的值都可以是null或undefined。

您可以将字段设置为可选的，而不是可空的。

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;
}

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';
}

type Nullable<T> = {
  [P in keyof T]: T[P] | null;
};

然后你就可以用它了

Nullable<Employee>

这样，您仍然可以像在其他地方一样使用Employee界面

type MyProps = {
  workoutType: string | null;
};