How to declare a type as nullable in TypeScript?

typescript nullable
Amitabh picture Amitabh · Jun 20, 2013 · Viewed 285.5k times · Source

I have an interface in TypeScript.

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

I would like to make salary as a nullable field (Like we can do in C#). Is this possible to do in TypeScript?

Answer

Ryan Cavanaugh picture Ryan Cavanaugh · Jun 20, 2013

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

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

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:

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';
}

Related questions

Can an optional parameter be null in TypeScript?

According to this article, when strict null checking is enabled in TypeScript, you cannot assign null or undefined to a variable unless it is explicitly allowed with a union. // required value let req: string; req = "Something"; // OK req = null; // Error …

Read More
How to convert a string to number in TypeScript?

Given a string representation of a number, how can I convert it to number type in TypeScript? var numberString: string = "1234"; var numberValue: number = /* what should I do with `numberString`? */;

Read More
Can't bind to 'ngModel' since it isn't a known property of 'input'

I've got the following error when launching my Angular app, even if the component is not displayed. I have to comment out the <input> so that my app works. zone.js:461 Unhandled Promise rejection: Template parse errors: Can't …

Read More