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[];
Typescript tuple spread type
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2 Answers
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
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
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