JavaScript React

Watching MobX Observables with the Reaction Function

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We can create Observable objects with MobX. To watch it for changes, we can use the reaction function.

In this article, we’ll look at how to use the reaction function to watch for value changes in MobX Observables.


We can use the reaction function as follows:

reaction(() => data, (data, reaction) => { sideEffect }, options?)

The reaction function is a variation on autorun that gives more fine-grained control on how an observable will be tracked.

The side-effect won’t be run directly when the observable is created. It’ll run only after the data expression returns a new value for the first time.

Any observables that are accessed while executing the side effect won’t be tracked.

reaction returns a disposer function.

The second function passed to reaction will retrieve 2 arguments when it’s run. The first argument is the value returned by the data function. The second argument is the current reaction during execution.

The side effect only reacts to data that was accessed in the data expression. It’ll only be triggered when the data returned by the expression has changed. This means that we have to produce things we need on our side effects.


The third argument of reaction is an options object that can take the following optional options:

  • fireImmediately — boolean that indicates the effect function should trigger immediately after the first run of the data function. This is false by default.
  • delay — number of milliseconds that can be used to debounce the effect function. If it’s zero then no debouncing will happen
  • equals — a compare function that’ll be used to compare the previous and next values produced by the data function. The effect function will only be invoked if the function returns false . Default is comparer.default .
  • name — a string that’s used as the name for the reaction
  • onError — a function that’ll handle the errors of this reaction, rather than propagating them
  • scheduler — set a custom scheduler to determine how re-running the autorun function should be scheduled


We can use the reaction function as follows:

import { reaction, observable } from "mobx";
const todos = observable([  
    title: "eat"  
    title: "drink"  

  () => => todo.title),  
  titles => console.log(titles.join(", "))  

todos.push({ title: "sleep" });

In the code above, we created a new observable array using the observable function, which we assigned to the todos constant.

Then we call the reaction function, where we pass a callback to return an array which has the title strings from the todos observable array as the first argument.

In the second argument, we get the titles array returned from the function in the first argument, and the console.log the titles array joined together with join .

Then we call push on it, as we did in the last line, the new comma-separated string will be logged with the console.log since we changed it. It won’t log the value when it’s initially created.


We can use the reaction function to watch for observable variable changes.

It won’t watch for the value when it’s initially created.

reaction takes a callback to watch the value and return something we want from it. The second argument takes a value that takes the returned value from the first function and then we can perform some side effects on it.

The 3rd argument takes an object that takes a variety of options.

By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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