// Tutorial //

Flow Type Aliases

Published on July 11, 2017
    Default avatar

    By Matthew Garcia

    Flow Type Aliases

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    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 {
    
    }
    

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    About the authors
    Default avatar
    Matthew Garcia

    author

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