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In TypeScript, how do you define a property that can be absent but will never be null or undefined?

November 16, 2024

A service is calling our API. It can call us with these types of JSON bodies:

const missingProperty = {}
const value = { foo: "bar" }

It will never call us with these bodies:

const nullish = { foo: null }
const undefinedish = { foo: undefined }

Is there a way to define a type that works this way? Here’s what I’ve tried, but it allows all possibilities:

type Foo = {
  foo?: string,
}

type Foo2 = {
  foo: string | undefined,
}

This doesn’t allow an empty object:

type Foo = {
    foo: string | never,
}

const missing: Foo = {} // Unwanted error
const value: Foo = { foo: "bar" }  
const nullish: Foo = { foo: null } // Wanted error
const undefinedish: Foo = { foo: undefined } // Wanted error

>Solution :

You’re looking for the exactOptionalPropertyTypes option.

If true :

interface MyObject {
  myProperty?: string; // Optional, and when present, it must be a string.
}

const obj1: MyObject = {}; // Valid: myProperty is absent
const obj2: MyObject = { myProperty: "Hello" }; // Valid: myProperty is a string
const obj3: MyObject = { myProperty: undefined }; // Invalid: undefined is not assignable
const obj4: MyObject = { myProperty: null }; // Invalid: null is not assignable

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