Maybe type

functionalProgramming fpTerm monad

Definition from Haskel

A maybe is a monad
The Maybe type is defined as follows:

``` data Maybe a = Just a | Nothing

```

Represent values that may or may not exist. It can be useful if you have a record field that is only filled in sometimes. Or if a function takes a value sometimes, but does not absolutely need it. (this is from elm)

    { name : String
    , age : Maybe Int
    }

tom = { name = "Tom", age = Just 42 }
sue = { name = "Sue", age = Nothing }

Maybe satisfies the type equation $F X = 1 + X$ , where the functor $F$ takes a set to a point plus that set.

source

Definition from flow

It's common for JavaScript code to introduce "optional" values so that you have the option of leaving out the value or passing null instead.

Using Flow you can use Maybe types for these values. Maybe types work with any other type by simply prefixing it with a question mark ? such as ?number as a sort of modifier.

Maybe types accept the provided type as well as null or undefined.

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Maybe type in Typescript

It is something or nothing.
In JS nothing can be: null or undefined. Due to this is better to define the Nothing as undefined.

If an API returns null, then could use something like this:

function convertToMaybe(value) {
  return value ?? undefined;
}

Or in TS terms, could be:

type Maybe<T> = NonNullable<T> | undefined;

This is a Generic type of the name T.

Generics act as a placeholder for a future type not yet decided. Generics let us pass in a type, much like how we pass arguments to a function. We are then using the Generic T to pass into the NonNullable type, which basically ensures that T can't be null or undefined. We are then using the union type operator, |, to also allow undefined as an allowed value.