typescript promise 输入然后

Question

为什么然后块 session 可以为空以及如何修复它？
Typescript sandbox.

type X = {id: string, session: {smt: string} | null}

const x1 = new Promise<X>(resolve => {
    resolve('smth' as unknown as X)
})

x1.then((user)=>{
    if (!user.session) throw new Error('e')

    user.session.smt // ok

    return user
})
.then(user=>{
    user.session.smt // why session is nullable again?
})
.catch(e=> e)

Answer 1

TypeScript 不知道用户对象在此期间未被修改。例如，其他人可能已经做了:

x1.then((user)=>{
    user.session = null; // that's allowed
})