The Add node adds a key and a value (key-value pair) to a Map, thereby increasing the length of the Map. When adding a key-value pair to a Map, the node checks whether the added key is equal to an existing key in the Map.
If the new key is equal to a key that's already in the Map, the existing value associated with the key will be overwritten with the new value. After this node has completed its operation, the key is guaranteed to be associated
with its corresponding value until a subsequent mutation of the Map.