Reactjs: how to get item value into a variable from axios data received

Question

I am confused as how to get a value of an item from a data returned by axios function. I've looked at many examples, but only avail are returning value using map, such as below.

render() {
  var student = this.state.student;
  return (
    student.map( d => (
      ..
      <td>{d.grade}
      ..
  );
)

But what I'm trying to do is store the value in a variable for manipulation. For example below, I want to extract the student grade:

class Student extends Component {
  state = { student: [], };
  
  componentDidMount() {
    axio.get(window.$API_URL).then(res=>this.setState({stduent: res.data}));
    var student = this.state.student;
    var grade = student.grade;                       //** doesn't work
    var grade = this.getGrade(student);              //** doesn't work
    var grade = getPropertyValue(student, "grade");  //** doesn't work
    ....
  }

  getGrade(student) {
    student.map(d=>{return(d.grade)});
  }

  getPropertyValue(obj, dataToRetrieve) {
    return dataToRetrieve
      .split('.')
      .reduce(function(o, k) {
        return o && o[k];
      }, obj)
  }

  render() { ....  }
}