如何在已知关联元素的情况下从JavaScript对象中获取值？

let obj = {
  key: 'value'
};

let value = obj.key; // 'value'

let obj = {
  key: 'value'
};

let key = 'key';
let value = obj[key]; // 'value'

// 或者使用动态键名
let dynamicKey = 'key';
let value = obj[dynamicKey]; // 'value'

let obj = {
  key: 'value'
};

let nonExistentValue = obj.nonExistentKey; // undefined

if (obj.hasOwnProperty('nonExistentKey')) {
  let value = obj.nonExistentKey;
} else {
  console.log('Key does not exist');
}

let notAnObject = 'I am a string';

// 这将抛出错误
// let value = notAnObject.key;

// 正确的做法是先检查类型
if (typeof notAnObject === 'object' && notAnObject !== null) {
  let value = notAnObject.key;
} else {
  console.log('Not an object');
}