Vue calculate and write a new property to the object (this.$set is not a function)?

Question

Vue.js

Hello, the question is the following: there is an array of objects that needs to be drawn as a list,
but before drawing, you need to calculate a new property and enter it into the object
, I do it like this

computed: {
    calculatedTables:function () {
      for (let table in this.tables) {
        this.$set(table, 'datetime',555)
        console.log(table)
      }
}

I get an error in the browser console

(in promise) TypeError: this.$set is not a function

when inserting through Object.assign all the rules except that the property is not rendered
what to do?

Answer 1

computed - must only perform a calculation and return a result, it must not mutate the data.
Use methods or watch to change data

Answer 2

You can't mutate in computed
If you need to add an object field, you can use .map

return this.tables.map(table => ({ ...table, datetime: 555 }))