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';
}

为了更像c#，可以这样定义Nullable类型:

type Nullable<T> = T | null;

interface Employee{
   id: number;
   name: string;
   salary: Nullable<number>;
}

奖金:

为了使Nullable表现得像一个内置Typescript类型，在根源文件夹的global.d.ts定义文件中定义它。这个路径适合我:/src/global.d.ts

在我看来，联合类型在这种情况下是最好的选择:

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 };

编辑:为了使其按预期工作，您应该在tsconfig中启用strictNullChecks。

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];

可空类型可以调用运行时错误。 所以我认为使用编译器选项strictNullChecks并将数字| null声明为类型是很好的。同样在嵌套函数的情况下，虽然输入类型是null，编译器不知道它会破坏什么，所以我建议使用!(感叹号)。

function broken(name: string | null): string {
  function postfix(epithet: string) {
    return name.charAt(0) + '.  the ' + epithet; // error, 'name' is possibly null
  }
  name = name || "Bob";
  return postfix("great");
}

function fixed(name: string | null): string {
  function postfix(epithet: string) {
    return name!.charAt(0) + '.  the ' + epithet; // ok
  }
  name = name || "Bob";
  return postfix("great");
}

参考。 https://www.typescriptlang.org/docs/handbook/advanced-types.html#type-guards-and-type-assertions

type WithNullableFields<T, Fields> = {
  [K in keyof T]: K extends Fields 
    ? T[K] | null | undefined
    : T[K]
}

let employeeWithNullableSalary: WithNullableFields<Employee, "salary"> = {
  id: 1,
  name: "John",
  salary: null
}

或者你可以关闭strictNullChecks;)

反过来说:

type WithNonNullableFields<T, Fields> = {
  [K in keyof T]: K extends Fields
    ? NonNullable<T[K]>
    : T[K]
}