那就是手写个 get
```js
function get(obj, path, def) {
var fullPath = path
.replace(/\[/g, '.')
.replace(/]/g, '')
.split('.')
.filter(Boolean);

return fullPath.every((step)=>{
return !(step && (obj = obj[step]) === undefined);
}) ? obj : def;
}

let a = {
b: {
c: {
d: 1
}
}
}

let z = 'c.d'
// console.log(a.b[z]) // undefined


console.log(get(a,'b.c.d'))


```