// Tutorial //

Objects as Maps in Flow

Published on July 13, 2017
    Default avatar
    By Matthew Garcia
    Developer and author at DigitalOcean.
    Objects as Maps in Flow

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    Despite the advent of the Map type, plain objects is an incredibly common way of representing a map. To support this paradigm, Flow offers a very flexible way to express this.

    Where Object Maps are Useful

    Let’s say you want to write a function that totals the cost of a grocery list. Invoking it will look something like:

    // This would return 9.5 (4 + 3 + 2.5).
    getTotalCost({
      eggs: 4,
      fruitSnacks: 3,
      toothpaste: 2.5,
    });
    

    The list can have any number of items, so you can’t explicitly declare each item as a property. Also, you want to make sure it only takes numbers, so:

    getTotalCost({
      eggs: 4,
      toothpaste: 2.5,
      milk: 'Too much',
    });
    

    …would give a type error.

    Declaring a Type as an Object Map

    Like a lot of things with Flow, it’s pretty simple. Just declare a typed key in brackets and give it a type:

    type GroceryList = {
      // The keys of the object will be strings.  The values will be numbers.
      [name: string]: number,
    };
    
    // From there, use it as any other type.
    function getTotalCost(groceryList: GroceryList): number {
      // Take the value of each property, return the sum.
    }
    

    getTotalCost will accept an object whose string keys have number values and give a type error for anything else:

    // This works.
    getTotalCost({
      eggs: 4,
      fruitSnacks: 3,
      toothpaste: 2.5,
    });
    
    // Also okay.
    getTotalCost({});
    
    // Nope.
    getTotalCost({
      eggs: 4,
      toothpaste: 2.5,
      milk: 'Too much',
    });
    

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    About the authors
    Default avatar
    Developer and author at DigitalOcean.

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