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How to (not) use decorator syntax
How to (not) use decorators

How to (not) use decorators

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Using decorators in MobX is optional. This section explains how to use them, or how to avoid them.

Advantages of using decorator syntax:

  • Minimizes boilerplate, declarative.
  • Easy to use and read. A majority of the MobX users use them.

Disadvantages of using decorator syntax:

  • Stage-2 feature
  • Requires a little setup and transpilation, only supported with Babel / Typescript transpilation so far

You can approach using decorators in two ways in MobX

  1. Enable the currently experimental decorator syntax in your compiler (read on)
  2. Don't enable decorator syntax, but leverage the MobX built-in utility decorate to apply decorators to your classes / objects.

Using decorator syntax:

import { observable, computed, action } from "mobx"

class Timer {
    @observable start =
    @observable current =

    get elapsedTime() {
        return this.current - this.start + "milliseconds"

    tick() {
        this.current =

Using the decorate utility:

import { observable, computed, action, decorate } from "mobx"

class Timer {
    start =
    current =

    get elapsedTime() {
        return this.current - this.start + "milliseconds"

    tick() {
        this.current =
decorate(Timer, {
    start: observable,
    current: observable,
    elapsedTime: computed,
    tick: action,

For applying multiple decorators on a single property, you can pass an array of decorators. The decorators application order is from right to left.

import { decorate, observable } from "mobx"
import { serializable, primitive } from "serializr"
import persist from "mobx-persist"

class Todo {
    id = Math.random()
    title = ""
    finished = false
decorate(Todo, {
    title: [serializable(primitive), persist("object"), observable],
    finished: [serializable(primitive), observable],

Note: Not all decorators can be composed together, and this functionality is just best-effort. Some decorators affect the instance directly and can 'hide' the effect of other decorators that only change the prototype.

The observer function from mobx-react is both a decorator and a function, that means that all these syntax variants will work:

class Timer extends React.Component {
	/* ... */

const Timer = observer(class Timer extends React.Component {
	/* ... */

const Timer = observer((props) => (
	/* rendering */

Enabling decorator syntax

If you want to use decorators follow the following steps.


Enable the compiler option "experimentalDecorators": true in your tsconfig.json.

Babel 6: using babel-preset-mobx

A more convenient way to setup Babel for usage with mobx is to use the mobx preset, that incorporates decorators and several other plugins typically used with mobx:

npm install --save-dev babel-preset-mobx


    "presets": ["mobx"]

Babel 6: manually enabling decorators

To enable support for decorators without using the mobx preset, follow the following steps. Install support for decorators: npm i --save-dev babel-plugin-transform-decorators-legacy. And enable it in your .babelrc file:

    "presets": ["es2015", "stage-1"],
    "plugins": ["transform-decorators-legacy"]

Note that the order of plugins is important: transform-decorators-legacy should be listed first. Having issues with the babel setup? Check this issue first.

Babel 7

Install support for decorators: npm i --save-dev @babel/plugin-proposal-class-properties @babel/plugin-proposal-decorators. And enable it in your .babelrc file:

    "plugins": [
        ["@babel/plugin-proposal-decorators", { "legacy": true }],
        ["@babel/plugin-proposal-class-properties", { "loose": true }]

Note that the legacy mode is important (as is putting the decorators proposal first). Non-legacy mode is WIP.

Decorator syntax and Create React App (v1)

  • Decorators are not supported out of the box in create-react-app@1.*. To fix this, you can either use the decorate utility, eject, or use the react-app-rewired package.

Decorator syntax and Create React App (v2)

  • Decorators are only supported out of the box when using TypeScript in create-react-app@^2.1.1. In older versions or when using vanilla JavaScript use either the decorate utility, eject, or the customize-cra package.

Disclaimer: Limitations of decorator syntax:

The current transpiler implementations of decorator syntax are quite limited and don't behave exactly the same. Also, many compositional patterns are currently not possible with decorators, until the stage-2 proposal has been implemented by all transpilers. For this reason the scope of decorator syntax support in MobX is currently scoped to make sure that the supported features behave consistently accross all environments

The following patterns are not officially supported by the MobX community:

  • Redefining decorated class members in inheritance trees
  • Decorating static class members
  • Combining decorators provided by MobX with other decorators
  • Hot module reloading (HMR) / React-hot-loader might not work as expected

Decorated properties might not be visible yet on class instances as own property until the first read / write to that property occurred.

(N.B.: not supported doesn't mean it doesn't work, it means that if it doesn't work, reported issues will not be processed until the official spec has been moved forward)