detectChangedProps

detectChangedProps(oldState, newState, path, ancestors): string[]

Computes the list of **dotted leaf paths** that changed between two values. The algorithm performs a deep structural comparison with special handling for: - **Primitives / null** → treated as leafs (change = current path; two NaNs are equal) - **Date** → compares getTime() - **RegExp** → compares source and flags - **Arrays** → if lengths differ, the whole array path is marked changed; otherwise compares element-by-element producing paths like "items.0.title" - **Objects** → compares by the **union of keys**, recursing into shared keys and marking added/removed keys as changed at their **full path** Cycles are handled by tracking the (old, new) pairs currently on the **recursion path** (added on entry, removed on unwind). A pair is skipped only when it is a genuine ancestor of itself (a real cycle) — a pair that merely appears again at a *sibling* path (legitimate aliasing, e.g. the same object referenced from two keys) is still diffed, so real changes at the second site are never dropped.

Parameters

Returns

string[] — An array of **dotted leaf paths** that changed. Paths use "." as a separator and indices for arrays (e.g., "todos.0.title"). If nothing changed, returns [].

detectChangedProps(
  { user: { name: 'Ada', age: 37 } },
  { user: { name: 'Grace', age: 37 } }
);
// => ['user.name']

detectChangedProps(
  { items: [{ title: 'A' }, { title: 'B' }] },
  { items: [{ title: 'A+' }, { title: 'B' }] }
);
// => ['items.0.title']

detectChangedProps({ nums: [1,2] }, { nums: [1,2,3] });
// => ['nums']

detectChangedProps(new Date(0), new Date(0), 'createdAt');      // => []
detectChangedProps(new Date(0), new Date(1), 'createdAt');      // => ['createdAt']
detectChangedProps(/a/i, /a/i, 'pattern');                      // => []
detectChangedProps(/a/i, /a/g, 'pattern');                      // => ['pattern']