Write code assuming inputs (arrays, objects, DOM nodes) may contain undefined, null, or “empty slots/holes”, and normalize them early with explicit defaults.

Standards

• Guard reads: when locating DOM nodes or accessing optional properties, use null-safe patterns (e.g., optional chaining) and avoid assuming existence.
• Normalize missing fields: if a required field might be absent (e.g., zone), assign a deterministic default ("general").
• Handle array holes explicitly: if your logic iterates arrays, don’t assume all indices contain values—treat holes like missing entries and decide whether to skip/compact them.
• Test the “empty” cases: include tests for both undefined values and array holes (e.g., [, ]), not just one.

Example

function groupByZone(actions) {
  return actions.reduce((acc, action) => {
    const zone = action?.item?.zone ?? 'general'; // default when missing/undefined
    (acc[zone] ??= []).push(action);
    return acc;
  }, {});
}

// Null-safe DOM access
const el = document.querySelector('.container > div');
console.assert(el?.classList.contains('output'));
console.assert(el?.classList.contains('hide'));

// Example for array-hole aware compaction
function compactFragments(arr) {
  return arr.filter(Boolean); // removes undefined/null and also skips holes
}