post_mutation.js

const resolvers = {
  Mutation:{
          createPost(parent,args,ctx,info){
              const userExists = users.some((user)=> user.id === args.author)

              if(!userExists){
                  throw new Error('User does not exist!')
              }
              
              //use this
              const post = {
                  id: uuidv4(),
                  title: args.title,
                  body: args.body,
                  published: args.published,
                  author:args.author
              }
              //OR
              //use object spread operator
              const post = {
                  id: uuidv4(),
                  ...args
              }

              posts.push(post)
              return post
          }
    }
 }