Limits of D3.js .enter() selection with key functions

Limits of D3.js .enter() selection with key functions

The goal is to guarantee that each new datum added to a data array results in exactly one corresponding DOM element when using D3's data‑join pattern with a key function.

Constraints arise when the key function may produce duplicate values, undefined, or when dealing with very large datasets that could affect internal selection caching.

• How does D3 treat duplicate keys returned by the key function when computing the .enter() selection?
• What is the behavior of .enter() when the key function returns undefined or null for a datum?
• Are there any performance or correctness limits to consider when the data array contains tens of thousands of items and a key function is used?