> For the complete documentation index, see [llms.txt](https://alexhisen.gitbook.io/mobx-recipes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alexhisen.gitbook.io/mobx-recipes/be-aware-of-short-circuited-code-in-reactions.md).

# Be aware of short-circuited code in reactions

MobX reactions (i.e. observers, autoRuns, reactions) and computeds are re-evaluated whenever any observables they access are changed. The caveat is that the reaction in question must have previously accessed the observable for it to be re-evaluated. Short-circuit `if`expressions can often create a situation where, in the initial evaluation, the observable you intended to be observed was not accessed and so future changes to it do not result in the re-evaluation of the reaction.

For example:

```javascript
import { autorun, observable} from 'mobx';

class MyClass {
  @observable isOnline = false;
  @observable hasError = false;
}

const obj = new MyClass();

autorun(() => {
  if (!obj.isOnline || obj.hasError) {
    // do something
  }
});
```

The first time the above `autorun`executes, it will not access the `hasError`observable. So, if later, `hasError`is set to `true`without `isOnline`first getting set to `true`, the `autorun`will not re-evaluate. (Once `isOnline`becomes `true`, only then this `autorun`will actually observe `hasError`.)

To prevent this from happening, access all observables that can change independently outside of short-circuited expressions, i.e.:

```javascript
import { autorun, observable} from 'mobx';

class MyClass {
  @observable isOnline = false;
  @observable hasError = false;
}

const obj = new MyClass();

autorun(() => {
  const isOnline = obj.isOnline;
  const hasError = obj.hasError;
  if (!isOnline || hasError) {
    // do something
  }
});
```