tassign

This is a simple wrapper for fixed-arity, subset-typed, non-mutating Object.assign.

Why?

This came about when writing Redux reducers in TypeScript. Since reducers must not mutate the passed in state, a common pattern is to 'set' store properties using Object.assign():

interface IMyState {
  foo: Number;
}
 
function myReducer(state: IMyState, action: Action) {
  switch(action.type) {
    case SET_FOO_ACTION: 
      return Object.assign({}, state, { foo: 3 });
  }
  
  return state;
}

This works well enough. Object.assign to an empty object produces a new object with the new value of foo. However I wasn't happy with it from a type-enforcement perspective.

The reason is that for a reducer use case, I would like to enforce that the reducer can only set known properties of IMyState.

Using the regular, intersection-based Object.assign typings, this is perfectly legal:

interface IMyState {
  foo: Number;
}
 
function myReducer(state: IMyState, action: Action) {
  switch(action.type) {
    case SET_FOO_ACTION: 
      // No TS error - I would like there to be.
      return Object.assign({}, state, { foo: 3, bar: 'wat' });
  }
  
  return state;
}

Formally, Object.assign(t:T, u:U) returns a type which is the intersection of T & U, meaning that properties can be added onto the assignee. This is reasonable in the general case for Object.assign, but it's not what I want in a reducer.

In a reducer, I want assign(t:T, u:U) to enforce a subset rule: the return value should be an instance of T, and only subsets of T's type are legal values for U. In short I want to make sure you can only assign members of U to the object returned by the reducer.

So, here's a utility that handles the typings the way I want in a reducer:

interface IMyState {
  foo: Number;
}
 
function myReducer(state: IMyState, action: Action) {
  switch(action.type) {
    case SET_FOO_ACTION: 
      // Returns a new object with the type of the first argument.
      // Type of the second argument must be a subset of the type of the first argument.
      
      // Fails: bar is not a member of IMyState.
      return tassign(state, { foo: 3, bar: 'wat' });
  }
  
  return state;
}