CODEWITHSUNDEEP
typescript
MEMark
MEMark

Reputation: 1529

Typescript tuple spread type

What is the purpose and meaning of defining a type like this..?
type AccountDetails1 = [AccountDetails, ...AccountDetails[]];

Is it equivalent to this union type..?
type AccountDetails2 = AccountDetails | AccountDetails[];

Upvotes: 2

Views: 161

Answers (2)

Umberto
Umberto

Reputation: 599

type AccountDetails1 = [AccountDetails, ...AccountDetails[]];

It means that variables from type AccountDetails1 must be an array of AccountDetails, and

type AccountDetails2 = AccountDetails | AccountDetails[];

Can be either an AccountDetails or an array of AccountDetails

Upvotes: 1

Terry
Terry

Reputation: 66123

It simply means that an array with the type of AccountDetails1 must be an array of AccountDetails with a minimum length of 1. The first part of the tuple indicates that the first element in the array must be of type AccountDetails, and the second part of the tuple means that the array may contain zero or more AccountDetails item in the array.

In short:

type AccountDetails1 = [AccountDetails, ...AccountDetails[]];

const arr1: AccountDetails1 = [AccountDetails]; // OK
const arr2: AccountDetails1 = [AccountDetails, AccountDetails]; // OK
const arr3: AccountDetails1 = []; // ERROR

In other words, type AccountDetails2 = AccountDetails | AccountDetails[]; means that it can be either of type AccountDetails or an array of AccountDetails with zero or more members:

type AccountDetails2 = AccountDetails | AccountDetails[];

const arr1: AccountDetails2 = [AccountDetails]; // OK
const arr2: AccountDetails2 = [AccountDetails, AccountDetails]; // OK
const arr3: AccountDetails2 = []; // OK
const obj1: AccountDetails2 = AccountDetails; // OK

Upvotes: 3

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