map函数不将数据返回到渲染

Question

userid(uid){
    this.state.userid.map((user)=>{
      if(uid===user._id){
        console.log(user.username)
        return user.username
      }
    })
  }

<div style={{width:"80%"}}>

{this.state.posts.map(post=>
  <div key={post._id.toString()}>
    <div className="modal-content"><br/>
    <a >{this.**userid**(post.user_id)}</a>        
    <b>Title: {post.title}</b><br/>
    <div><b>Message: </b>{post.message}</div>

我从返回渲染中调用用户id，但它没有返回，有没有办法返回