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Objects as Maps in Flow

Published on July 13, 2017
Matthew Garcia

By Matthew Garcia

Objects as Maps in Flow

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 author

Matthew Garcia
Matthew Garcia
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