How to finish all fetch before executing next function in React?

Question

Using ReactJS, I have two different API points that I am trying to get and restructure: students and scores. They are both an array of objects.

My goal is : first, get students and scores, and second, with students and scores saved in state, I will modify them and create a new state based on students and scores state. In short, I have 3 functions: getStudents, getScores, and rearrangeStudentsAndScores. getStudents and getScores need to finish before rearrangeStudentsAndScores can run.

My problem is: sometimes rearrangeStudentsAndScores will run before getScores would complete. That messed rearrangeStudentsAndScores up. But sometimes it would complete. Not sure why it works 50% of the time, but I need to make it work 100% of the time.

This is what I have to fetch students and scores in my Client file:

function getStudents(cb){
    return fetch(`api/students`, {
        headers: {
            'Content-Type': 'application/json',
            'Accept': 'application/json'
        }
    }).then((response) => response.json())
    .then(cb)
};

function getScores(cb){
    return fetch(`api/scores`, {
        headers: {
            'Content-Type': 'application/json',
            'Accept': 'application/json'
        }
    }).then((response) => response.json())
    .then(cb)
};

I then combined them together:

function getStudentsAndScores(cbStudent, cbScores, cbStudentsScores){
    getStudents(cbStudent).then(getScores(cbScores)).then(cbStudentsScores);
}

In my react app, I have the following:

getStudentsAndScores(){
    Client.getStudentsAndScores(
        (students) => {this.setState({students})},
        (scores) => {this.setState({scores})},
        this.rearrangeStudentsWithScores
    )
}

rearrangeStudentsWithScores(){
    console.log('hello rearrange!')
    console.log('students:')
    console.log(this.state.students);
    console.log('scores:');
    console.log(this.state.scores);        //this returns [] half of the time
    if (this.state.students.length > 0){
        const studentsScores = {};
        const students = this.state.students;
        const scores = this.state.scores;
        ...
    }
}

Somehow, by the time I get to rearrangeStudentsWithScores, this.state.scores will still be [].

How can I ensure that this.state.students and this.state.scores are both loaded before I run rearrangeStudentsWithScores?

Answer 1

Your code mixes continuation callbacks and Promises. You'll find it easier to reason about it you use one approach for async flow control. Let's use Promises, because fetch uses them.

// Refactor getStudents and getScores to return  Promise for their response bodies
function getStudents(){
  return fetch(`api/students`, {
    headers: {
      'Content-Type': 'application/json',
      'Accept': 'application/json'
    }
  }).then((response) => response.json())
};

function getScores(){
  return fetch(`api/scores`, {
    headers: {
      'Content-Type': 'application/json',
      'Accept': 'application/json'
    }
  }).then((response) => response.json())
};

// Request both students and scores in parallel and return a Promise for both values.
// `Promise.all` returns a new Promise that resolves when all of its arguments resolve.
function getStudentsAndScores(){
  return Promise.all([getStudents(), getScores()])
}

// When this Promise resolves, both values will be available.
getStudentsAndScores()
  .then(([students, scores]) => {
    // both have loaded!
    console.log(students, scores);
  })

As well as being simpler, this approach is more efficient because it makes both requests at the same time; your approach waited until the students were fetched before fetching the scores.

See Promise.all on MDN