How to define a function in javascript reduce function?

I have an object student, it has property id, name, groupName. allStudents is the array of student objects, I want to turn it to an id to student map, but for each student, I want to generate a new property "label", if the student has groupName, label value is "name + groupName", otherwise it is name. So I write below code, it works:

const idsToStudents = allStudents.reduce((tempMap, student) => {
    const getStudentLabel = (student) => {
        if (student.groupName) {
            return [student.name, `(${student.groupName})`].join(' ');
        }
        return student.name;
    };
    const studentLabel = getStudentLabel(student);
    return {
        ...tempMap,
        [student.id]: { ...student, label: studentLabel}
    };
}, {});

I define getStudentLabel function inside reducer function, is there a better way to do this instead of declare getStudentLabel function again and again in the reducer function? You can ignore what exactly getStudentLabel does, just think it takes each person as parameter and return something based on person, is there a way to define the function only once, but still I can call it for each person in the reducer?

Thank you!

You are passing student as a parameter to the function, so you don't need to declare it inside the reduce. This would work as well:

const getStudentLabel = (student) => {
  if (student.groupName) {
    return [student.name, `(${student.groupName})`].join(' ');
  }
  
  return student.name;
};

const idsToStudents = allStudents.reduce((tempMap, student) => {
  const studentLabel = getStudentLabel(student);
  return {
      ...tempMap,
      [student.id]: { ...student, label: studentLabel}
  };
}, {});

And you can also shorten the code a bit:

const getStudentLabel = ({ name, groupName }) => groupName
  ? `${name} (${groupName})`
  : name;

const idsToStudents = allStudents.reduce((tempMap, student) => ({
  ...tempMap,
  [student.id]: { ...student, label: getStudentLabel(student) }
}), {});