The blog of developer Timo Zöller

Using a generic and typesafe set method with mobx-state-tree in TypeScript

Managing state in a JavaScript or a React.js application is complex problem. My personal preferred solution is mobx-state-tree which is build on the also great MobX. One of my favourite features is the necessity to call an explicit action to change a value. For the most part this action is bound to the model. It looks like this:

const model = types.model({
  name: types.string
}).actions(self => ({
  setName(name: string) {
    self.name = name;

It's typesafe and it's a nice way to create domain specific actions. But it can become cumbersome if you have a lot of different fields which are independently of each other. You could create a set method for every field, but … no.

So after failing to find some way by myself I asked in the mobx-state-tree spectrum chat and we managed to find a solution together:

import { cast, SnapshotIn, types } from 'mobx-state-tree';

const model = types
  .model("model", {
    text: types.string,
    number: types.number,
    bool: types.boolean,
    optional: types.optional(types.string, "default"),
    complex: types.maybe(
        prop: types.string
  .actions(self => ({
      K extends keyof SnapshotIn<typeof self>,
      T extends SnapshotIn<typeof self>
    >(key: K, value: T[K]) {
      self[key] = cast(value);

This is just great because it's typesafe. You can check the sandbox with example code here. But also because this was an awesome online colaboration experience.

Thanks to ingowonner for trying again and again and providing the final touch by using the cast function.