Map versus Object

Moddable SDK 9.0.0

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Map versus Object

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Map versus Object

You can use both Objects and Maps for collections of items indexed by a key.

Maps are relatively new to JavaScript (over 10 years now!) so it is still common to see Objects used as a map.

// using Object
let obj = new Object();
obj["one"] = 1;
obj["two"] = 2;
obj["three"] = 3;

// using Map
let map = new Map();
map.set("one", 1);
map.set("two", 2);
map.set("three", 3);

Maps are specified to take a more-or-less constant time to access an element. That means that adding, looking up, and deleting items in a map should always take a relatively short period of time. Objects have no such requirement. This means that in Embedded JavaScript you will likely get more predicable performance from a Map than an object. But, to provide this performance guarantee maps use a bit more memory per element than an object.

Which should you use? If the number of items is relatively small, say a few dozen, use whatever you are most comfortable coding. If the number of items may be large, a Map is probably your best bet. As always, to be certain measure the performance and memory using both.

For a collection of items that is not indexed by a key, scripts commonly use Array. Set is a good alternative. The tradeoffs are similar to using a Map instead of an Object. Another benefit of Set is that removing an item requires less code.

// Using Array
let array = [];
array.push("one", "two", "three");

let index = array.indexOf("two");
if (index >= 0)
	array.splice(index, 1); 
	
let hasThree = array.includes("three");
// => hasThree === true

// Using Set
let set = new Set();
set.add("one");
set.add("two");
set.add("three");

set.delete("two");

let hasThree = set.has("three");
// => hasThree === true