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

Answer 2

I believe you need to wrap your functions in arrow functions. The functions are being invoked as the promise chain is being compiled and sent to the event loop. This is creating a race condition.

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

I recommend this article for additional reading: We Have a Problem with Promises by Nolan Lawson

And here's a repo I made that has an example for each of the concepts talked about in the article. Pinky Swear