D3 data binding matches an array of data to the DOM elements in a selection. Each datum either updates an existing element, enters without an element, or leaves an element without matching data. Use .join() for the usual create-update-remove pattern; add a key function when records should keep their visual identity after reordering.
What data binding does
Think of a data join as comparing two collections: the elements already selected and the data supplied now. D3 pairs them and makes the unmatched cases available as enter and exit selections. These names describe the result of one join, not permanent categories of DOM nodes.
Calling .data(data) associates values with selected elements and returns the update selection. D3 stores each bound value on its element in __data__, making it available if you select that element again; the D3 selection.data reference calls this data “sticky.” The binding defines which data and elements correspond; it does not, by itself, create missing elements.
Make a basic join with .join()
Start with an SVG selection and an array such as const data = [{name: "A", value: 12}, {name: "B", value: 20}];. A concise join looks like this:
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svg.selectAll("circle")
.data(data)
.join("circle")
.attr("r", d => d.value)
.attr("cx", d => x(d.name))
.attr("cy", d => y(d.value));Here, selectAll("circle") selects the current circles in the SVG. .data(data) compares them with the new array, and .join("circle") appends a circle for each entering datum, keeps the updating circles, and removes exiting ones. The selection returned by .join() combines entering and updating elements, so the attribute setters that follow apply to both.
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Change the array length and run the join again: extra data produce new circles, while extra old circles are removed. Keep the same number of records but change their values, and the attribute setters update the existing marks.
When enter, update and exit need different treatment
Most joins need no separate callbacks. Use them when entering, updating and exiting elements need distinct setup or behavior:
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svg.selectAll("circle")
.data(data, d => d.id)
.join(
enter => enter.append("circle").attr("r", 0),
update => update,
exit => exit.remove()
)
.attr("r", d => radius(d.value));The enter callback creates circles and gives them an initial radius; the update callback keeps existing circles; the exit callback removes unmatched circles. The final radius setter applies to the merged enter-and-update selection. The D3 joining reference also supports transitions in these callbacks: if an enter or update callback returns a transition, D3 merges the underlying selections.
In the older explicit pattern, call .enter() to handle missing elements and .exit() to handle elements without data. If an operation should apply to both newly entered and updated elements, merge those selections with .merge(update). Otherwise it is easy to configure only new elements and leave existing ones stale.
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Without a key function, D3 pairs data and elements by position: the first datum with the first element, the second datum with the second element, and so on. This is appropriate when order stays stable and position itself carries the intended identity.
If records can move, be sorted, or be refreshed as newly created JavaScript objects, use a stable identifier so an element continues to represent the same record. The key function is evaluated for existing elements and incoming data; its returned key is a string identifier. For records with unique IDs, for example, use .data(data, d => d.id).
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| Matching approach | How D3 matches | Best fit |
|---|---|---|
| Index join | By position in the selection and data array | Order is stable, or position is the intended identity |
| Key join | By the string identifier returned by the key function | Records may reorder or be reconstructed but should retain their element identity |
Make keys unique within the relevant selection group. D3 sends duplicate keys on existing elements to exit and duplicate keys in incoming data to enter, rather than matching those duplicates as one-to-one identities. A stable key also matters when refreshed data contains new object instances: matching by a field such as an ID can connect records across arrays even though the JavaScript objects themselves differ. See the Square Intro to D3 data-binding tutorial for a beginner-friendly illustration.
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D3 performs joins independently within each selection group. If there is one group, pass its array directly to .data(). When each parent has its own child data, pass a function that returns the array for each group, often from the parent datum.
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For example, after binding rows to table-row elements, bind each row’s values to its cells with .data(d => d). In a grouped SVG where each parent datum has a children array, the analogous pattern is .data(d => d.children). This is why a flat array is not a substitute when every parent needs different child data. The D3 data-joining documentation demonstrates the row-and-cell pattern.
Quick Recap
Common mistakes to avoid
- Expecting
.data()to create elements: it defines the join. Use.join()or handle.enter()to create missing nodes. - Updating only entering elements: shared attributes should be applied to the merged selection returned by
.join(), or to enter and update selections merged explicitly in the older pattern. - Ignoring exiting elements:
.join()removes them by default; supply an exit callback when they need different handling. - Using position as identity after a reorder: use a unique, stable key when a visual element should follow a particular record.
- Passing one flat array to every nested group: use a data function when child data depends on each parent datum.
- Reusing duplicate keys: duplicate element keys exit and duplicate incoming-data keys enter, so they do not provide dependable identity.
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