Difference between string[] and [string]

Question

Consider the following Typescript example. The first line results in an error 'type undefined[] is not assignable to type [string]'. The last two lines do compile.

let givesAnError: [string] = [];
let isOK: string[] = [];
let isAlsoOK: [string] = ["foo"];

How do you have to interprete the type definition [string] in Typescript?

Answer 1

The first (givesAnError) and last (isAlsoOK) are tuples, and the second (isOK) is an array.

With arrays all of your elements are of the same type:

let a: string[];
let b: boolean[];
let c: any[];

But with tuples you can have different types (and a fixed length):

let a: [string, boolean, number];
let b: [any, any, string];

So:

a = ["str1", true, 4]; // fine
b = [true, 3, "str"]; // fine

But:

a = [4, true, 3]; // not fine as the first element is not a string
b = [true, 3]; // not fine because b has only two elements instead of 3

It's important to understand the the javascript output will always use arrays, as there's no such thing as tuple in js.
But for the compilation time it is useful.

Answer 2

Straight to it

string[] // n-length array, must only contain strings

[string] // must be 1-length array, first element must be a string