```javascript
componentDidMount() {
let a = { b: 1, c: 2, d: [1, 2, 3] }
console.log(a?.d?.length ?? 0)
}
````
在babel配置中添加兩個包即可：
"@babel/plugin-proposal-optional-chaining",
"@babel/plugin-proposal-nullish-coalescing-operator",
