This error keeps bugging me while I'm trying to get a grab on typescript. Can anyone explain what is the difference between the two 'array'? Is any of this not an array? If not, what is it then?
[ts] Type 'AppointmentItemData[]' is not assignable to type '[AppointmentItemData]'.
Property '0' is missing in type 'AppointmentItemData[]'.
Your first type (AppointmentItemData[]) is an array of AppointmentItemData elements, while your second type is a tuple, that may only ever contain a single AppointmentItemData. Quoting from the docs:
Tuple types allow you to express an array where the type of a fixed number of elements is known, but need not be the same. For example, you may want to represent a value as a pair of a
stringand anumber:// Declare a tuple type let x: [string, number]; // Initialize it x = ["hello", 10]; // OK // Initialize it incorrectly x = [10, "hello"]; // Error
Typescript allows you to encode that "this variable should only every have a 3 element array in it, where the first element is a number, the second is a string, and the third is a Promise that will eventually resolve to something shaped like a Lizard", whereas if it didn't the only way to encode [number, string, Promise<Lizard>] would be {}[] (that is, roughly, an Object[]).
[1, 'hi', someSalamander] as an object (e. g. {0: 1, 1: 'hi', 2: someSalamander}), but it is an array. That is, thingTypedAsTuple instanceof Array === true. As to how you'd use it - anywhere you want to carry around a series of things as opposed to a set of named things. Some examples include [success, error], [value, validations], and [x, y, z].