javascript - 为什么 typescript 不知道我检查了对象类型？

Question

我想构建这样一个函数:

const recursionProxy = <T extends object>(subject: T) =>
  new Proxy(subject, {
    get(target, key: keyof T) {
      const nestedSubject = target[key];

      if (typeof nestedSubject === "object") {
        return recursionProxy(nestedSubject);
      }

      return nestedSubject ?? target._ ?? "Message not set";
    },
  });

但是在 recursionProxy(nestedSubject); 行下有一个错误说

[i] Argument of type 'T[keyof T]' is not assignable to parameter of type 'object'.

为什么 typescript 不考虑 if staement，在 if 语句中 nestedSubject 是对象类型

Answer 1

如果您使用类型谓词，它似乎确实有效:

const isObject = (f: any): f is object => {
   if(typeof f === "object") return true;
return false;
}
const recursionProxy = <T extends object>(subject: T) =>
  new Proxy(subject, {
    get(target, key: keyof T) {
      const nestedSubject = target[key];

      if (isObject(nestedSubject)) {
        return recursionProxy(nestedSubject);
      }

      return nestedSubject ?? target._ ?? "Message not set";
    },
  });

Link

如果需要，也可以在谓词中添加空检查:

const isObject = (f: any): f is object => {
  if(f === null) return false;
   if(typeof f === "object") return true;
return false;
}