Using ECMAScript 6 Maps

Originally published in the A Drip of JavaScript newsletter.

In last week's drip I discussed using Object.create(null) in order to simplify creating key-value maps in JavaScript. But even with that approach, objects aren't a substitute for a full-fledged map data type. For example:

var heroesNemesis = Object.create(null);

var greenLantern = { name: "Green Lantern" };
var sinestro = { name: "Sinestro" };

heroesNemesis[greenLantern] = sinestro;

// Outputs: { name: "Sinestro" }

var wonderWoman = { name: "Wonder Woman" };

// Outputs: { name: "Sinestro" }

Why is our object reporting Wonder Woman's nemesis as Sinestro? In JavaScript, an object's keys can only be strings. If you try to supply something else, the JavaScript engine will try to coerce it into a string with toString.

And the default toString implementation for an object returns "[object Object]" regardless of what properties the object possesses. So unless we decide to start monkeying around with Object.prototype (generally a bad idea) our "map" object can't distinguish between different objects as keys.

Fortunately, ECMAScript 6 (the next version of JavaScript) includes a new Map data type which allows us to use any valid JavaScript value as a key.

var heroesNemesis = new Map();

var greenLantern = { name: "Green Lantern" };
var sinestro = { name: "Sinestro" };

heroesNemesis.set(greenLantern, sinestro);

// Outputs: { name: "Sinestro" }

var wonderWoman = { name: "Wonder Woman" };

// Outputs: undefined

As you can see, Map gives us the result we'd expect. To use Map our main interfaces are set which accepts a key and associated value to store, and get which accepts a key and returns the associated value.

When checking to see whether a given key matches, Map uses the "same value" algorithm. So even if two objects look identical, only the original can be used to retrieve it's associated value. For example:

var clonedLantern = { name: "Green Lantern" };

// Outputs: undefined

And you can determine how many elements have been stored in a Map by checking its size property.

// Outputs: 1

This basic set of functionality for Map is supported in the latest versions of Chrome and FireFox, as well as IE11. In future drips we'll look into more advanced features of Map.

Thanks for reading!

Joshua Clanton

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