// Tutorial //

Flow Type Aliases

Published on July 11, 2017
    Default avatar
    By Matthew Garcia
    Developer and author at DigitalOcean.
    Flow Type Aliases

    When written solely as literals, Flow has a tendency to get very verbose very quickly. It is often useful to declare repeated types as type aliases.

    Declaring a Type Alias

    Let’s say you’re writing functions to manipulate todo lists:

    // Add a new todo item to `todos` and return the new item.
    function addTodo(
      todos: Array<{id: number, name: string, completed: boolean}>,
      name: string,
    ): {id: number, name: string, completed: boolean} {
      // Implementation omitted for brevity.
    }
    
    // Get a todo item from `todos` by `id`.
    function getTodo(
      todos: Array<{id: number, name: string, completed: boolean}>,
      id: number,
    ): {id: number, name: string, completed: boolean} {
      // Implementation omitted for brevity.
    }
    
    // Get all incomplete todo items in `todos`.
    function getIncompleteTodos(
      todos: Array<{id: number, name: string, completed: boolean}>,
    ): Array<{
      id: number,
      name: string,
      completed: boolean,
    }> {
      // Implementation omitted for brevity.
    }
    

    You probably noticed that {id: number, name: string, completed: boolean} gets repeated a lot. Besides being really verbose, this has other problems; if you want to add something to your todo items (dateCompleted, assignedTo, et cetera), you have to edit every single instance of {id: number, name: string, completed: boolean}, which is tedious and error-prone.

    So, instead, let’s alias that as its own type, TodoItem. It’s pretty simple, like declaring a variable:

    type TodoItem = {id: number, name: string, completed: boolean};
    

    From there, you can use TodoItem as a type as you would any other type.

    // The type alias.
    type TodoItem = {id: number, name: string, completed: boolean};
    
    // Add a new todo item to `todos` and return the new item.
    function addTodo(todos: Array<TodoItem>, name: string): TodoItem {
      // Implementation omitted for brevity.
    }
    
    // Get a todo item from `todos` by `id`.
    function getTodo(todos: Array<TodoItem>, id: number): TodoItem {
      // Implementation omitted for brevity.
    }
    
    // Get all incomplete todo items in `todos`.
    function getIncompleteTodos(todos: Array<TodoItem>): Array<TodoItem> {
      // Implementation omitted for brevity.
    }
    

    Exporting Type Aliases

    TodoItem can be used in other files, too. Just mark it as an export like this:

    // todos.js
    export type TodoItem = {id: number, name: string, completed: boolean};
    // someOtherFile.js
    import type {TodoItem} from './todos';
    
    function somethingThatTakesATodoItem(item: TodoItem): void {
    
    }
    

    Want to learn more? Join the DigitalOcean Community!

    Join our DigitalOcean community of over a million developers for free! Get help and share knowledge in our Questions & Answers section, find tutorials and tools that will help you grow as a developer and scale your project or business, and subscribe to topics of interest.

    Sign up
    About the authors
    Default avatar
    Developer and author at DigitalOcean.

    Still looking for an answer?

    Was this helpful?
    Leave a comment