Also, this merge operation overwrites any cells property on the row, which could potentially lead to unexpected behavior when trying to access this via the injected row:

<script>
  import { DataTable } from "carbon-components-svelte";

  const headers = [
    { key: 'organism', value: "Organism" },
    { key: 'cells', value: "Cells" }
  ];

  const rows = [
    { id: 0, organism: "Elegans - Adult male", cells: 1031 },
    { id: 1, organism: "Elegans - Adult hermaphrodite", cells: 959 },
    { id: 2, organism: "Elegans - Hatched larvae", cells: 558 },
  ];
</script>

<DataTable {headers} {rows} title="row.cells unexpected">
  <span slot="cell" let:row let:cell>
    {#if cell.key == 'cells'}
      {row.cells}
    {:else}
      {cell.value}
    {/if}
  </span>
</DataTable>

In #1179, the implementation changed so that rows is not modified. cells is no longer written to the rows prop.

However, this is still an issue if using slotted cells, as row.cells will still override the cells property. This is addressed by 5d61929.

@brunnerh Can you provide a simple repo of your use case to illustrate the first problem?

@metonym Do you mean the type being lost?

This illustrates two use cases when using classes: REPL

• Function on prototype is lost ("Show" button)
• Row object identity is lost ("Delete" button)

I currently cannot access the original project using proxies, but this should be pretty similar: REPL
The cell value is read correctly from the underlying proxy but the row cannot be used to read or update the value.
To fixe the marked errors, the underlying data or the original rows have to be accessed instead.
E.g. row[cell.key][row.property] => data[cell.key][row.property]