n8n: Cannot read properties of undefined (reading '...') [line N]

This is a JavaScript TypeError raised inside a Code node (or the older Function node). The code tried to read a property from something that is undefined. n8n appends the line number in square brackets, so [line 2] tells you exactly where.

In n8n it almost always means the incoming data has a different shape than the code expects: a field name is different, a level of nesting is missing, an item has no json because the previous node returned nothing, or the node is in "Run Once for All Items" mode while the code is written for a single item (or the reverse). AI and HTTP nodes are common sources, because their output structure varies.

What is not wrong: n8n itself. The fix is in the code or the input data.

// Run Once for All Items
return $input.all().map(item => ({
  json: { count: (item.json?.results ?? []).length }
}));

Validate inputs at the start of Code nodes, handle empty and missing data explicitly, and prefer built-in nodes such as Edit Fields where possible, since they tolerate shape changes better.