ypeError: Cannot read properties of undefined (reading 'length')

这个错误通常是由于尝试访问未定义或未初始化的变量或属性而引起的。以下是一些可能导致此错误的常见情况：

1. 访问未定义的变量或属性

let arr;
console.log(arr.length); // TypeError: Cannot read properties of undefined (reading 'length')

2. 访问未初始化的变量或属性

let obj = {};
console.log(obj.prop); // undefined
console.log(obj.prop.length); // TypeError: Cannot read properties of undefined (reading 'length')

3. 函数未正确返回值

function getArray() {
  // 没有返回值
}
console.log(getArray().length); // TypeError: Cannot read properties of undefined (reading 'length')

4. 对于异步操作，尝试在操作完成之前访问结果

let arr;
fetch('https://example.com/data.json')
  .then(response => response.json())
  .then(data => {
    arr = data;
  });
console.log(arr.length); // TypeError: Cannot read properties of undefined (reading 'length')