Creating a Map

A Map is a basic mapping of keys to values. Maps are different from objects in that their keys can be anything (primitive values as well as objects), not just strings and symbols. Iteration over Maps is also always done in the order the items were inserted into the Map, whereas the order is undefined when iterating over keys in an object.

To create a Map, use the Map constructor:

const map = new Map();

It has an optional parameter, which can be any iterable object (for example an array) which contains arrays of two elements – first is the key, the seconds is the value. For example:

const map = new Map([[new Date(), {foo: "bar"}], [document.body, "body"]]);
//                      ^key          ^value          ^key        ^value