setWith(state, ...overrides)

Offered as a utility for applying state updates, setWith is kind of like Object.assign, except that its behavior has been fine-tuned for use when writing Reducers. The notable differences from stock Object.assign are that:

  1. This function may return its first argument state if the functional result would otherwise shallow-equal state. This is important to Redux workflows that don't want to create unnecessary copies.
  2. The TypeScript typing of this function is more specific to the state-update workflow: for state objects that have no index signature, overrides must be a Partial of the state's type. This makes usages of setWith more resilient than plain assign to naming refactors, for instance.
  3. This function must be called with at least two arguments, as otherwise setWith(obj) === obj always.
  4. This function is careful about prototype-handling, and will return an object without a prototype if the input state has no prototype.
  5. The always-empty first arg is omitted.


import { TypedReducer, setWith as sw } from "redoodle";
import { AppState } from "./AppState";
import { SetStatus } from "./actions/SetStatus";

const reducer = TypedReducer.builder<AppState>();

// Applies the deeply nested copy-on-write `state.statuses[] = payload.status.
// If the old status and the new end up being the same, this entire handler is a no-op.
// Each of the assignments are type-safe.
reducer.addHandler(SetStatus.TYPE, (state, payload) => {
  return sw(state, {
    statuses: sw(state.statuses, {
      []: payload.status

If the input state to setWith is unmodified by the operation (that is, it's shallow-equal to its old value), the same state reference itself is returned by setWith() to encourage reference equality between state updates.

TypeScript definition

function setWith<S, K extends keyof S>(
  state: S,
  override: (Partial<S> & Pick<S, K>),
  ...addlOverrides: (Partial<S> & Pick<S, K>)[],
): S;

function setWith<S extends Record<string, any>>(
  state: S,
  override: S,
  ...addlOverrides: S[],
): S;

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