Define Equality Semantics

Ensure algorithmic helpers that use data structures (e.g., deep equality, key intersections) have an explicit correctness contract and are implemented with runtime-compatible structures.

• For deep equality: verify the algorithm enforces all required invariants (e.g., same keys present in both objects, not just equal lengths). Clearly decide whether special values follow native === semantics or “collection” semantics (e.g., treating NaN as equal like Set).
• For intersections/maps/sets: don’t assume Set/custom map types are available or supported across all target runtimes. Prefer native Map when broadly supported, or implement a fallback/intersection that avoids Set when legacy environments matter.

Example (avoid Set for key intersection):

// Compatible intersection without Set
function sharedKeys(a: Record<string, unknown>, b: Record<string, unknown>) {
  const out: string[] = [];
  for (const key in a) {
    if (Object.prototype.hasOwnProperty.call(b, key)) out.push(key);
  }
  return out;
}

Apply this by (1) writing down the intended semantics (especially for equality) and (2) choosing the simplest compatible data structure/approach for the supported JavaScript environments.