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TypeError: Cannot read properties of undefined (reading 'map')

function List({ items }) {
  return items.map((i) => <li>{i}</li>);
}

Why it fails: items is undefined on the first render, before your data has loaded, so .map() has nothing to run on.

Suggested fix

function List({ items = [] }) {
  return items.map((i) => <li key={i}>{i}</li>);
}

TypeError: can only concatenate str (not "int") to str

count = 3
print("Found " + count + " files")

Why it fails: Python won't join text and a number with +. Convert the number to text, or use an f-string.

Suggested fix

count = 3
print(f"Found {count} files")

Query returns every row instead of one customer

SELECT * FROM orders
WHERE customer_id = customer_id;

Why it fails: The column is compared with itself, which is always true. Compare it with the value you want, passed as a parameter.

Suggested fix

SELECT * FROM orders
WHERE customer_id = $1;