How deeply does D3.js in Action, Third Edition cover key function?

Explains positional matching, uid-based matching, and integration with D3 data() and join(), without descending into API internals.

Key function

As explained in D3.js in Action, Third Edition

A key function computes stable identity keys that allow data-bound elements to be matched with corresponding data items across updates.

A key function computes a stable identity for each datum so D3 can keep data-bound elements matched with the same data across updates. Without one, D3 matches elements and data by position: the first circle receives the first updated datum, the second receives the second, and so on. With a key such as each cetacean’s uid, circles that remain visible continue representing the same cetacean. section 7.4.2

Keep filtered scatterplot points attached to their data

Pass the key function as the second argument to data(): innerChart .selectAll("circle") .data(updatedData, d => d.uid) .join("circle"); When filters change the cetacean dataset, circles that remain on the scatterplot keep their associated data instead of changing identity. New points enter, existing points update, and removed points exit.

How D3.js in Action, Third Edition covers it

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Also explained in

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The Quick Python Book, Fourth EditionA key function is a callable that maps each item to a value used to determine that item's ordering during sorting.

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Related concepts

maximumused by · A maximum is the greatest value in a set under a specified ordering, or the greatest value permitted by a specified constraint.
sorting algorithmused by · A sorting algorithm is a procedure that rearranges elements of a collection into an order determined by an ordering criterion.
data()configures · data() is a D3 selection method that associates an array of data values with the selected document elements.
lambdaimplemented by · A lambda is a language construct that defines an unnamed function or callable, often as a value.
sortedused by · sorted is a Python built-in function that returns a new list containing an iterable’s elements in sorted order without modifying the original iterable.

This explainer was generated by Manning's knowledge graph from the text of D3.js in Action, Third Edition. The section links point to the parts of the book it draws on.