Type aliases

When your types become more complex or when you want to reuse the same definition in several places, you can create a type alias:

// Source file: src/types_advanced.js

type simpleFlag = number | boolean;

type complexObject = {
 id: string,
 name: string,
 indicator: simpleFlag,
 listOfValues: Array<number>
};

After defining types in this fashion, you can just use them anywhere, even in the definition of new types, as we did in complexObject, where we defined a field to be of the previously defined simpleFlag type:

// Source file: src/types_advanced.js

let myFlag: simpleFlag;

let something: complexObject = {
 id: "B2209",
 name: "Anna Malli",
 indicator: 1,
 listOfValues: [12, 4, 56]
};

Type aliases can even be generic, as we'll see in the next section. You can also export types from a module, and import them for usage anywhere; we'll get to that in the Working with libraries section.