Here are the TLDR common methods of defining arrays in TypeScript.
[string] = Tuple (fixed size array)
string = Array (most common array)
When defining an array in TypeScript you might think it is okay to define it using
[string]. I have made this mistake several times & I find others making it often as well. This is actually defining a tuple, which is probably not the array you are wanting.
The TypeScript handbook does a great job of defining a tuple, which is common in many other languages.
"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." https://www.typescriptlang.org/docs/handbook/basic-types.html#tuple
let x: [string, number]; x = ["hello", 10];
A common use of tuples is storing output after running a function, especially for short status messages like writing to a file or submitting an HTTP request.
let status: [string, number] = submitContent(text, urlPath); console.log(status); //(2) ["error", 400]
On a side note, the above example would be a great situation to use destructuring.
let x: string; x = ["hello", "world"]
In cases where the array is mixed with different types you would use the
Array <string> version. Otherwise you may get an error stating Typescript cannot Invoke an expression whose type lacks a call signature. Here's an example of an array with multiple types.
let x: Array<string | number> x = ["hello", "world", 2]
This second version is common if your array consists of different types of objects. For example:
let inventory: Array<Boat, SpaceShip, Wagon>
You might notice that the list of types above are all vehicle types that could possibly share a parent type called
Transportation. Depending on your exact problem, you could solve it with Generics. Another possible solution is to define a type like the following:
type Vehicle = Boat | SpaceShip | Wagon let inventory: [Vehicle]
In short, use the
Array<string | number> version if you don't know how many items will be in your Array. Use the tuple
[string, number] version for small arrays with a fixed number of items that always have the same type in the order specified.