Debugging with Console & DevTools

Which debugging approach would be most efficient for identifying the specific problem in these functions?
let results = [];

// This should process items but something's wrong
function processItems(items) {
  for (let i = 0; i < items.length; i++) {
    let result = processItem(items[i]);
    results.push(result);
  }
  return results;
}

// This function has bug that's affecting the results
function processItem(item) {
  // Complex processing
  return item.value * 2;
}
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