Incorrect element transitions during data reordering in d3.selection.join

When utilizing the d3.selection.join() method in D3.js v5+, the library manages the lifecycle of DOM elements by mapping a data array to a selection. By default, this mapping is performed based on the index of the element within the array.

A conflict arises when the underlying dataset is reordered or filtered. If the .data() method is called without a specific key function, D3 binds the new data to existing elements based on their position rather than a unique identifier. This behavior can cause visual artifacts where elements appear to morph into unrelated data points during transitions instead of moving to their new coordinates.

Technical Constraints

• Targeting D3.js v5+ ES6 module implementation.
• Requirement for persistent element identity across data updates.
• Avoidance of index-based joining for dynamic datasets.

What is the precise mechanism for implementing a key function within the .data() call to ensure join() maintains object constancy? How does the internal __data__ property update when a key function is provided versus when it is omitted?