Vue.js 3 - replace/update reactive object without losing reactivity

Though Boussadjra Brahim's solution works its not the exact answer to the question.

In the sense that reactive data can not be reassigned with = but their is a way to reassign the reactive data. It is Object.assign.

Therefore this should work

    setup(){
        const formData = reactive({})
    
        onMounted(() => {
          fetchData().then((data) => {
            if (data) {
              Object.assign(formData, data) // equivalent to reassign 
            }
          })
        })
      }

Note: The solution is the Best way when your reactive object empty or always contains same keys; but if e.g formData has key x and data does not have key x then after Object.assign, formData will still have key x, so not strictly not reassigning.

reactive should define a state with nested fields that could be mutated like :

 setup(){
    const data= reactive({formData :null })

    onMounted(() => {
      fetchData().then((data) => {
        if (data) {
          data.formData = data 
        }
      })
    })

  }

or use ref if you just have one nested field:

  setup(){
    const formData = ref({})

    onMounted(() => {
      fetchData().then((data) => {
        if (data) {
          formData.value = data 
        }
      })
    })

  }