The idea behind weak is pretty far fetched for most developers who have strong, rigid of how maps and key/value pairs and references work. data structures mental models In this article, I try to explore the workings of weak structures by observing their behaviour, without being bothered about their implementation. Concept Refreshers References and cloning In the code below, we create a variable that holds a to value that is stored somewhere in the memory. We then create another variable that is a clone of . Since holds a primitive value, it is cloned into . x reference 111 y x x deep y var x = 111;var y = x; y *= 2; console.log(x, y);// 111 222 In the following snippet, we do everything exactly the same as before, except for the first assignment — assigning an object instead of a primitive. x As a result, holds a new reference to the same object that references. This is cloning. y x shallow var x = {a: 111};var y = x; y.a *= 2; console.log(x.a, y.a);// 222 222 Garbage Collection The basic principle of garbage collection is that if some memory location can’t be accessed then it must be removed from the memory. Consider the following snippet: x = {...};y = x; delete x;y = ...; When we created and , we were holding 2 references to the same thing. When we , we lose one reference but we can still access the object from . However, when we reassign , we lose all references to the object and it can’t be accessed anymore, but it still occupies some memory. x y delete x y y This is where a GC comes in handy. A garbage collector will notice this anomaly and will quickly “free” the occupied inaccessible memory. You can read more about this . here on MDN vs WeakMap Map Map First, let’s observe how a Map behaves in the wild. When we add a new key/value pair to a map, the map stores the mapping. Primitives will get deep cloned and object types will get a new reference pointing to them. obj = {};map = new Map(); map.set(obj, 123); console.log(map.get(obj)); // 123 console.log(map.size); // 1console.log(map.keys());// [ ] {} If we now delete or re-assign it, it will no lose its reference to . But the map still holds the key/value pair. This so happens because when a map stores a key/value pair, it (for both items). obj {} creates references to objects obj = {};map = new Map(); map.set(obj, 123); // de-reference `obj` from {}obj = ... console.log(map.get(obj)); // undefined console.log(map.size); // 1console.log(map.keys());// [ {} ] WeakMap In contrast, we can say that a WeakMap — meta reference. Here’s the same code with a WeakMap. creates reference to a reference obj = {};wMap = new WeakMap(); wMap.set(obj, 123);console.log(wMap.get(obj)); // 123 If we now delete/reassign the , we will . Without the a key, the key/pair record in the map is marked for GC and then the value meets the same fate. obj lose the key in a key/value pair Set and WeakSet A set is a collection of unique entities — just like the keys in a map. So if we ignore the “value” part of a map, we end up with a set. Similarly, if we ignore the second columns in the map diagram, we get our Set and WeakSet. Conclusion Now that we “know” that weak structures create meta-references, it makes perfect sense why we can’t iterate over them or check their size. If you were to now ask the question why primitives can’t be keys, I’d say that“I can’t answer that”. This question is related to what happens under the hood. You can read more about them and . here here In contrast, we can say that a WeakMap . creates reference to a reference
