Map.prototype.get()

const map = new Map();
map.set("bar", "foo");

console.log(map.get("bar"));
// : "foo"

console.log(map.get("baz"));
// : undefined

get(key)

const myMap = new Map();
myMap.set("bar", "foo");

console.log(myMap.get("bar")); // "foo" 
console.log(myMap.get("baz")); // undefined 

const arr = [];
const myMap = new Map();
myMap.set("bar", arr);

myMap.get("bar").push("foo");

console.log(arr); // ["foo"]
console.log(myMap.get("bar")); // ["foo"]