Generic Object Update Function in Typescript

Question

The updateObject function below takes in oldObject of type T and newValues of type Partial<T>, and then assigns any of the values present innewValues to oldObject.

My problem is that if newValues contains properties not present in oldObject, my compiler throws an error. How can I express to the compiler that I want newValues to be reduced to only the properties present in oldObject?

Problematic code

In this code, newValues is of type Partial<Point> and point is of type ThreatPoint.

updateObject(point, newValues);

Error Message

The error I receive says:

Argument of type '(point: ThreatPoint) => (ThreatPoint & Partial<ThreatPoint>) | (RoutePoint & Partial<...>)' is not assignable to parameter of type '(item: ThreatPoint) => ThreatPoint'.
  Type '(ThreatPoint & Partial<ThreatPoint>) | (RoutePoint & Partial<...>)' is not assignable to type 'ThreatPoint'.
      Types of property 'category' are incompatible.
        Type 'Category.ROUTE_POINT' is not assignable to type 'Category.THREAT'.

updateObject Function

static updateObject = <T>(oldObject: T, newValues: Partial<T>) => {
   // Encapsulate the idea of passing a new object as the first parameter
   // to Object.assign to ensure we correctly copy data instead of mutating
   return Object.assign({}, oldObject, newValues);
};

Types/Interfaces

export interface BasicPoint {
  category: Category;
  id: string;
  position: Coordinate;
  title: string;
  type: PointType;
}

export enum RoutePointType {
  START = 'START',
  FINISH = 'FINISH',
}

export enum ThreatPointType {
  OBSTACLE = 'OBSTACLE',
  RIVER = 'RIVER',
}

export type PointType = RoutePointType | ThreatPointType;

export enum Category {
  ROUTE_POINT = 'ROUTE_POINT',
  THREAT = 'THREAT',
}

export interface RoutePoint extends BasicPoint {
  category: Category.ROUTE_POINT;
  speed: number;
  type: RoutePointType;
}

export interface ThreatPoint extends BasicPoint {
  category: Category.THREAT;
  type: ThreatPointType;
}

export type Point = RoutePoint | ThreatPoint;

By the wat, you you want to avoid mutation, Object.assign is not the best choice because it mutates {} — captain-yossarian from Ukraine
– captain-yossarian from Ukraine, Commented Sep 15, 2021 at 19:55
#captain-yossarian All of my code has been added to the post now. — RouteMapper
– RouteMapper, Commented Sep 15, 2021 at 22:28
@captain-yossarian That code doesn't even execute when you hit Run. It doesn't compile. — RouteMapper
– RouteMapper, Commented Sep 16, 2021 at 23:17

Dario Fiore · Accepted Answer · 2021-09-15 18:44:54Z

1

I tried to manage your use case by coding something like that:

 static updateObject = <T extends {}, X extends T>(oldObject: T, newValues: Partial<X> ): T => {
      
      const newObjectValues = { ...newValues }

      const keysToInclude = Object.keys(oldObject);

      const newValuesKeys  = Object.keys(newValues)

      const newValuesFiltered = newValuesKeys
        .filter(key => !keysToInclude.includes(key))
        .forEach(key => delete newObjectValues[key as keyof T]);

      return Object.assign({}, oldObject, newValuesFiltered);

    };

Basically, I said to the typescript compiler that the function can receive an argument of X type which extends the property of T. Then, I created newValuesFiltered in order to obtain an object only with the keys of T.

answered Sep 15, 2021 at 18:44

Dario Fiore

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1 Comment

RouteMapper Over a year ago

This solution doesn't solve my problem. Using the code in my post, and your solution, I get this error: Type 'Category.ROUTE_POINT' is not assignable to type 'Category.THREAT | undefined'.

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