Typescript: Type that requires at least one property

user_8884948 0
Viewed 4807

Consider that I have the following type:

type SomeType = {
  propOne: any;
  propTwo: any;
  propThree: any;
}

The propOne is required, propTwo and propThree are optional but at least one of them is required. How can I define the type with that constraint?

// The following code is my expectation
let someVar1: SomeType = { propOne: 1, propTwo: "two" } //Okay
let someVar2: SomeType = { propOne: 1, propThree: "three" } //Okay
let someVar3: SomeType = { propOne: 1, propTwo: "two", propThree: "three" } //Okay
let someVar4: SomeType = { propOne: 1 } //Not Okay
typescript javascript
2 Answers

You can use the same trick even after your edit:

type SomeType = {
  propOne: number;
  propTwo?: string;
  propThree?: string;
} & ({
  propTwo: string;
} | {
  propThree: string;
})

let someVar1: SomeType = { propOne: 1, propTwo: "two" } //Okay
let someVar2: SomeType = { propOne: 1, propThree: "three" } //Okay
let someVar3: SomeType = { propOne: 1, propTwo: "two", propThree: "three" } //Okay
let someVar4: SomeType = { propOne: 1 } //Not Okay

function f(s: SomeType) {
  let p1 = s.propOne; // number
  let p2 = s.propTwo; // string | undefined
  let p3 = s.propThree; // string | undefined
}

It forces you to have at least one property at declaration time, and it allows you to use both properties of the type when using objects of that type.

Playground

user_43779320

Here is type that is union type of 2 objects:

type SomeType = {
    propOne: any;
} | {
  propTwo: any;
}

Typescript will require object's type to be compatible to one of type in this union. Here is solution in playground

Note: This is the simplest solution if you really have only two properties.

user_130933100
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