JavaScript Maps¶

Depending on the language, these can be called Maps, Dictionaries, or associative arrays. Although there are differences in how they are implemented, the basic idea is the same. In the same way that we can use an array to store multiple values within one variable, we can use a map to store multiple values within a single variable. Instead of accessing the values based on an index number, however, we assign an arbitrary key to access it’s associated value.

MDN Map Reference

To create a map and set values:

let englishToFrench = new Map();
englishToFrench.set("hello", "bonjour");
englishToFrench.set("goodbye", "au revoir");

To access a value in the map:

englishToFrench.get("goodbye");

To check if a key exists inside a map:

if (englishToFrench.has("goodbye")) {
        console.log(englishToFrench.get("goodbye"));
}

Note that we can save any type of data inside the value, not just a string. It can be a number, an object, an array, etc. A simple example would be to save coordinates (an x, y pair) into a map as an object…

for (let [key, value] of someMap) {
  // the key variable holds the value used to access something in the map
  // the value variable holds whatever the contents are

  // in other words, someMap.get(key) would return the same thing as value
}