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var students = [
  '阿部直博',
  '安西翔大郎',
  '伊藤司',
  '井上壮',
  '宇野陽太'
  // ...
];

var index = students.indexOf('井上壮');

if (index === -1) {
  window.alert('そんな生徒はいません');
}
else {
  window.alert((index + 1) + '番');
}

var array = [1, 2, 3, 3, 4, 5];

var index = array.indexOf(3);

console.log(index);
// 2

var index = array.indexOf(value, fromIndex);