ReactJs program to add in array of objects if name already doesnt exist and if it exist alert the name already exist?

The alert doesn't come up the first time i submit a repeated value and the alert doesn't show the name

const addName = (event) => {
  event.preventDefault()
  const nameObject = {
    name: newName
  }
  const temp = persons.filter(
    person => person.name === nameObject.name
  )

  if (temp === undefined || temp.length === 0) {
    setPersons(persons.concat(nameObject))
    setNewName('')  
  } else {
    console.log(nameObject.name)
    alert(`${nameObject.name} already exists in phonebook`)
  }  
}