What is the Unexpected side effect in computed property error?

Question

Vue.js

Good day!
Here is such a problem.

Here is the code:

data() {
    return {
      messagesforuser:[],
      newmessagesforchat: []
    };
  },
computed: {
  ...
    messagesforuserReverse: function() {
       return this.messagesforuser.reverse()
    }
  },

I think the point is that it is initially empty, but what to do?

Answer 1

You shouldn't change the data in computed properties, but you do - because the reverse method does not create a new array, but changes the original one. So before using it, the array should be copied:
return [...this.messagesforuser].reverse()