MobX Quick Start Guide

Structural equality

If the return value of a computed property is a primitive, it is easy to know when there is a new value. MobX compares the previous value of the computed property with the newly evaluated value and then fires notifications if they differ. Thus, value comparisons become important to ensure notifications are fired only on a real change.

For objects, this is not straightforward. The default comparison is done based on reference checks (using the === operator). This treats the objects as being different even though the values within them are exactly the same.

In the following example, the metrics computed property generates a new object every time the start or end properties change. Since the autorun (defined in the constructor) ...