我有以下任务

Given an array of strings, write a function that returns an object that groups each of the input array's elements by the first character.

我的代码如下。我收到一个错误（（用注释突出显示）说

TypeError: Cannot read property '0' of undefined

有人能向我解释一下为什么它说它未定义吗？

function groupBy(arr) { 
  var result = {};
  var container = []
  
  for (var i = 0; i < arr.length; i++) {
    var currentWord = arr[i];
    var firstChar = currentWord[0];
    var nextWord = arr[i + 1];
    var secondChar = nextWord[0]; // Error here

    if (firstChar === secondChar) {
      container.push(currentWord, nextWord)
    } else {
      container.push(currentWord)
    }

    result[firstChar] = container;
  }
  return result;
}
console.log(groupBy(['apple', 'cat', 'boat', 'card', 'bond']));

// returns { 'a': ['apple'], 'c': ['cat', 'card'], 'b': ['boat', 'bond'] }