Graphql查询始终返回null

如何解决Graphql查询始终返回null

我正在尝试使用graphql获取课程数据，但是服务器始终返回null作为响应这是我在文件server.js中的代码：

var express=require('express');
const { graphqlHTTP } = require('express-graphql')
var {buildSchema}=require('graphql');

//graphql schema
var schema=buildSchema(`
type Query {
    course(id: Int!): Course
    courses(topic:String!):[Course]

}
type Course {
    id: Int
    title: String
    author: String
    description:String
    topic:String
    url: String
}
`);

var coursesData = [
    {
        id: 1,title: 'The Complete Node.js Developer Course',author: 'Andrew Mead,Rob Percival',description: 'Learn Node.js by building real-world applications with Node,Express,MongoDB,Mocha,and more!',topic: 'Node.js',url: 'https://codingthesmartway.com/courses/nodejs/'
    },{
        id: 2,title: 'Node.js,Express & MongoDB Dev to Deployment',author: 'Brad Traversy',description: 'Learn by example building & deploying real-world Node.js applications from absolute scratch',url: 'https://codingthesmartway.com/courses/nodejs-express-mongodb/'
    },{
        id: 3,title: 'JavaScript: Understanding The Weird Parts',author: 'Anthony Alicea',description: 'An advanced JavaScript course for everyone! Scope,closures,prototypes,this,build your own framework,and more.',topic: 'JavaScript',url: 'https://codingthesmartway.com/courses/understand-javascript/'
    }
]

//root resolver
var root= {
    course:getCourse,courses:getCourses
};


var getCourse= function (args){
    var id=args.id;
    console.log("delila")
    return coursesData.filter(course=>{
        return course.id==id
    })[0]

}
var getCourses = function(args){
    if(args.topic){
        var topic=args.topic;
        return coursesData.filter(course=>
            course.topic===topic
        );

    }
    else return coursesData
    
}
//create an experess server and graphql endpoint
var app=express();
app.use('/graphql',graphqlHTTP({
    schema: schema,rootValue:root,graphiql:true
}));
app.listen(4000,()=>console.log("delila express graphql server running on localhost 4000"))

当我去localhost:4000/graphql获取我正在使用的数据时

query getSingleCourse($courseID: Int !){
  course(id:$courseID){
    title
    author
    description
    url
    topic
  }
}

{
  "courseID": 3
}

但是我一直在得到结果null。看图片

有人知道为什么会这样吗？服务器应返回id 3的课程，但显然我缺少某些内容

解决方法

您应该先定义函数表达式，然后再使用它们。这就是原因。

与函数声明不同，不会悬挂JavaScript中的函数表达式。创建函数表达式之前，不能使用它们：

请参见Function expression

例如

//...

var getCourse = function (args) {
  var id = args.id;
  console.log('delila');
  return coursesData.filter((course) => {
    return course.id == id;
  })[0];
};
var getCourses = function (args) {
  if (args.topic) {
    var topic = args.topic;
    return coursesData.filter((course) => course.topic === topic);
  } else return coursesData;
};

//root resolver
var root = {
  course: getCourse,courses: getCourses,};

//...