Tip 13: Update Key-Value Data Clearly with Maps

In this tip, you’ll learn how to use the Map object for key-value collections of frequently updated data.

In Tip 10, Use Objects for Static Key-Value Lookups, you learned that you should only use objects deliberately and not as a default collection. Now you’re going to get a chance to look at an alternative: Map.


Map is a special kind of collection that can do certain things very easily. The Mozilla Developer Network has a nice list of circumstances where Map is a better option for a collection than a plain object. I encourage you to read the full list, but this tip examines two specific situations:

  • Key-value pairs are frequently added or removed.

  • A key isn’t a string.

In the next tip, you’ll see another big advantage: using Map for iterating over collections. For now, you just need to be familiar with adding or removing values to maps.

Adding & removing key-value pairs: Example

First, think about what it means that key-value pairs are frequently added and removed. Consider a pet adoption website. The site has a list of all the adorable dogs that need homes. Because people have different expectations of their pets (some want big dogs, some like certain breeds), you’ll need to include a way to filter the animals.

You’ll start off with a collection of animals:

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