Fetch: set variable with fetch response and return from function [duplicate]

Question

I'm quite new with JavaScript and react. I have a callback from a component that gets a customer_name from a server given a id. The fetch works and the console.log prints the fullname correctly, but the customer_name in the last .then is not set, and the functions returns an empty string. Why is that?

// Gets the fullname of the customer from an id.
tj_customer_name(id) {
  let customer_name = '';

 fetch(`/customers/${id}.json`, {
   headers: API_HEADERS,
   credentials: 'same-origin'
 })
 .then((response) => {
   if(response.ok) {
     return response.json();
   } else {
     throw new Error('Server response wasn\'t OK');
   }
 })
 .then((json) => {
   customer_name = json.first_name.concat(' ').concat(json.last_name);
   console.log(customer_name);
 });
 return customer_name;
}

Answer 1

I think you don't understand Promises correctly. The return statement will be called before the Promise is resolved, thus returning empty string.

One way to tackle this, is to return the whole promise like this:

// Gets the fullname of the customer from an id.
tj_customer_name(id) {
  let customer_name = '';

  return fetch(`/customers/${id}.json`, {
    headers: API_HEADERS,
    credentials: 'same-origin'
  })
  .then((response) => {
    if(response.ok) {
        return response.json();
    } else {
        throw new Error('Server response wasn\'t OK');
    }
  })
  .then((json) => {
    return json.first_name.concat(' ').concat(json.last_name);
  });
}

or you can use the ES7 approach, using async/await like this

async function tj_customer_name(id) {
    const response = await fetch('some-url', {});
    const json = await response.json();

    return json.first_name.concat(' ').concat(json.last_name);
}

As you can see, the second approach is much cleaner and readable.

The result will be the same in the code which calls your function

tj_customer_name(1).then(fullName => {
    console.log(fullName);
});

or

async function something() {
    const fullName = await tj_customer_name(1);
    console.log(fullName);
}

Answer 2

Because fetch is asynchronous and returns a promise, which by its nature can only be observed asynchronously (using .then).

You should probably just return the promise chain you create in your function and return customer_name in the last .then callback of the chain:

// Gets the fullname of the customer from an id.
tj_customer_name(id) {

 // return the entire promise chain
 return fetch(`/customers/${id}.json`, {
   headers: API_HEADERS,
   credentials: 'same-origin'
 })
 .then((response) => {
   if(response.ok) {
     return response.json();
   } else {
     throw new Error('Server response wasn\'t OK');
   }
 })
 .then((json) => {
   const customer_name = json.first_name.concat(' ').concat(json.last_name);
   return customer_name; // return the customer_name here
 });
}

// later, use the function somewhere
this.tj_customer_name(21).then((customer_name) => {
    // do something with the customer_name
});

PS: Don't forget to add a .catch handler to handle potential network issues (see: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch#Checking_that_the_fetch_was_successful)