WeakMap: is a type of Map where only objects can be passed as keys. Primitive data type -- such are string, numbers, booleans, etc --- are not allowed.
let user = {}; let comment = {}; let mapSettings = new WeakMap(); mapSettings.set(user, "user"); maåSettings.set(comment, "comment"); console.log(mapSettings.set(user)); console.log(mapSettings.set(comment));
if:
mapSettings.set("title", "ES2015"); --> invalid value used as weap map key
Wekmap is not iterable, therefore, they can‘t be used with for...of.
WeakMaps are better with Memory:
let user = {}; // all objects occupy memory space
let userSatatus = new WeakMap();
userStatus.set(user, "logged"); // Objecct reference passed as key to the WeakMap
//...
someOtherFunction( user );
// Once it returns, user object can be garbage collected
Weakmaps don‘t prevent the garbage colector from collecting objects currently used as keys, but that are no longer referenced anywhere else in the system.
原文：http://www.cnblogs.com/Answer1215/p/5129041.html