Understanding the "Key not weakly held" error in JavaScript

Understanding the "Key not weakly held" error in JavaScript

The "Key not weakly held" error in JavaScript usually occurs due to issues related to garbage collection and memory management in iterables. When you are using WeakMap or WeakSet, this error might arise due to holding onto a reference to the key improperly, preventing it from being garbage collected. Let's see what this error means and what fixes are available for it.

In summary, WeakMap allows keys to be weakly held, meaning that when there are no other references to the key, it becomes eligible for garbage collection. If a key that is being attempted to be referenced in WeakMap is not held in the proper structure of your data, issues will arise.

For example, assume you are working with WeakMap and trying to store objects as keys. If these objects are no longer accessible, you might encounter this error. This is a common issue in code that needs careful handling of how to maintain references to those objects properly.

In this case, to resolve this issue, you need to ensure that objects intended as keys in WeakMap are correctly defined in the program and their relations to other factors are maintained correctly. Use appropriate constructors and utilize this error as a reminder for correct behaviors in coding.

Code Example

const weakMap = new WeakMap();
const obj1 = {};
const obj2 = {};

weakMap.set(obj1, "value1");

console.log(weakMap.get(obj1)); // value1

 // If we lose the reference of obj1
 // obj1 = null; // Uncommenting this line will allow garbage collection to remove obj1

Line by Line Explanation

const weakMap = new WeakMap();
In this line, we create a new WeakMap named weakMap which can hold weak references to keys.

const obj1 = {};
In this line, we define an empty object named obj1 that will be used as a key in weakMap.

const obj2 = {};
Here, we define another object named obj2 which will not currently be used, but can persist later.

weakMap.set(obj1, "value1");
With this line, we associate obj1 as a key with the value "value1" in weakMap.

console.log(weakMap.get(obj1));
Here, we retrieve the stored value for obj1 using the get method of weakMap, which will return "value1".

// obj1 = null;
If you do not comment this line, in this state we would remove the reference to obj1 and this possibility would allow garbage collection to occur. Thus, weakMap can no longer keep a reference to obj1 and this may lead to an error.