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What is the [number] syntax?

The [number] syntax is a TypeScript syntax that allows you to retrieve the type of an array's elements.

Example#1​

type Numbers = number[];
type Number = Numbers[number]; // type Number = number

Example#2​

type Strings = string[];
type String = Strings[number]; // type String = string

So, the [number] syntax is used to retrieve the type of a specific array element. In this case, it retrieves the type of the first array element (since number is 0 by default).