What does the syntax d._children = d.children; stand for in d3.js?

Question

In various examples on tree visualizations such as this collapsible tree example the syntax d._children = d.children; is used. For example in this code block fr

Answer 1

_children is just a temp variable that holds the children when they are hidden. When you click you are either taking children to null and storing the children in the temp variable, or, if children is already null, loading them from the temp variable.

Any temp variable could have been used. There is nothing special about _children. It is used to show an obvious relationship to children.