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Work with external services

To establish integrations with external services, you can use the fetch function in scripts. It uses the native Fetch API in the browser and a polyfill on the server, so its interface combines features available to them.

The On-Premises server can be located behind a proxy. In this case, it is necessary to specify the parameters of the proxy in the system server’s environment variables HTTP_PROXY and HTTPS_PROXY to work with external services.

Read more about it in the Install BRIX to MicroK8s behind a proxy article in the Help Center.

Example of using fetch():

const res = await fetch('https://my.server/api/products', {
    method: 'POST',
    headers: {
        Authorization: 'Bearer SOME-TOKEN-HERE',
    },
    body: JSON.stringify({
        name: 'New product',
        cost: 13.20,
    })
});
if (!res.ok) {
    // Processing an error with a response code ≥ 300
}
const product = await res.json();

A simple GET request can be done without passing additional parameters:

const res = await fetch(`https://my.server/api/products/${ Context.data.itemId }`);

As the request’s parameters, you can pass the request’s method FetchRequest.method, the request’s headers FetchRequest.headers as a simple object Record<string, string> and the request’s body FetchRequest.body as a string.

More about fetch()

Source: https://javascript.info/fetch

JavaScript can send network requests to the server and load new information whenever it’s needed.

For example, we can use a network request to:

  • Submit an order,
  • Load user information,
  • Receive latest updates from the server,
  • …etc.

There’s an umbrella term “AJAX” (abbreviated Asynchronous JavaScript And XML) for network requests from JavaScript. We don’t have to use XML though: the term comes from old times, that’s why that word is there. You may have heard that term already.

There are multiple ways to send a network request and get information from the server.

The fetch() method is modern and versatile, so we’ll start with it. It’s not supported by old browsers (can be polyfilled), but very well supported among the modern ones.

The basic syntax is:

let promise = fetch(url, options)
  • url – the URL to access.
  • options – optional parameters: method, headers etc.

Without options, this is a simple GET request, downloading the contents of the url.

The browser starts the request right away and returns a promise that the calling code should use to get the result.

Getting a response is usually a two-stage process.

**First, the promise, returned by fetch, resolves with an object of the built-in Response class as soon as the server responds with headers. **

At this stage we can check HTTP status, to see whether it is successful or not, check headers, but don’t have the body yet.

The promise rejects if the fetch was unable to make HTTP-request, e.g. network problems, or there’s no such site. Abnormal HTTP-statuses, such as 404 or 500 do not cause an error.

We can see HTTP-status in response properties:

  • status – HTTP status code, e.g. 200.
  • ok – boolean, true if the HTTP status code is 200-299.

For example:

let response = await fetch(url);

if (response.ok) { // if HTTP-status is 200-299
  // get the response body (the method explained below)
  let json = await response.json();
} else {
  alert(\"HTTP-Error: \" + response.status);
}

Second, to get the response body, we need to use an additional method call.

Response provides multiple promise-based methods to access the body in various formats:

  • response.text() – read the response and return as text,
  • response.json() – parse the response as JSON,
  • response.arrayBuffer() – return the response as ArrayBuffer (low-level representation of binary data).

For instance, let’s get a JSON-object with latest commits from GitHub:

let url = 'https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits';
let response = await fetch(url);

let commits = await response.json(); // read response body and parse as JSON

alert(commits[0].author.login);```

Or, the same without await, using pure promises syntax:

```js
fetch('https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits')
  .then(response => response.json())
  .then(commits => alert(commits[0].author.login));

To get the response text, await response.text() instead of .json():

let response = await fetch('https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits');

let text = await response.text(); // read response body as text

alert(text.slice(0, 80) + '...');

** Important: We can choose only one body-reading method.**

If we’ve already got the response with response.text(), then response.json() won’t work, as the body content has already been processed.

let text = await response.text(); // response body consumed
let parsed = await response.json(); // fails (already consumed)

Response headers

The response headers are available in a Map-like headers object in response.headers.

It’s not exactly a Map, but it has similar methods to get individual headers by name or iterate over them:

let response = await fetch('https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits');

// get one header
alert(response.headers.get('Content-Type')); // application/json; charset=utf-8

// iterate over all headers
for (let [key, value] of response.headers) {
  alert(`${key} = ${value}`);
}

Request headers

To set a request header in fetch, we can use the headers option. It has an object with outgoing headers, like this:

let response = fetch(protectedUrl, {
  headers: {
    Authentication: 'secret'
  }
});

…But there’s a list of forbidden HTTP headers that we can’t set:

  • Accept-Charset, Accept-Encoding
  • Access-Control-Request-Headers
  • Access-Control-Request-Method
  • Connection
  • Content-Length
  • Cookie, Cookie2
  • Date
  • DNT
  • Expect
  • Host
  • Keep-Alive
  • Origin
  • Referer
  • TE
  • Trailer
  • Transfer-Encoding
  • Upgrade
  • Via
  • Proxy-*
  • Sec-*

These headers ensure proper and safe HTTP, so they are controlled exclusively by the browser.

POST requests

To make a POST request, or a request with another method, we need to use fetch options:

  • method – HTTP method, e.g. POST,
  • body – the request body, one of:
    • a string (e.g. JSON-encoded),
    • FormData object, to submit the data as multipart/form-data.

The JSON format is used most of the time.

For example, this code submits user object as JSON:

let user = {
  name: 'John',
  surname: 'Smith'
};

let response = await fetch('/article/fetch/post/user', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json;charset=utf-8'
  },
  body: JSON.stringify(user)
});

let result = await response.json();
alert(result.message);

Please note, if the request body is a string, then Content-Type header is set to text/plain;charset=UTF-8 by default.

But, as we’re going to send JSON, we use headers option to send application/json instead, the correct Content-Type for JSON-encoded data.

Sending a file

We can also submit binary data with fetch using FormData and ArrayBuffer objects.

async function submit() {
    let data_req = await fetch('/external/data/to/load');
    let buffer = await data_req.arrayBuffer();
    let form = new FormData();
    form.append('file', buffer, 'filename.ext');
    let response = await fetch('/article/fetch/post/file', {
        method: 'POST',
        body: form
    });

    // the server respond with a confirmation and the size of the image
    let result = await response.json();
    alert(result.message);
}

Please note, here we don’t set Content-Type header manually because it is automatically set in multipart/form-data.

Summary

A typical fetch request consists of two await calls:

let response = await fetch(url, options); // resolves with response headers
let result = await response.json(); // read body as json

Or, without await:

fetch(url, options)
  .then(response => response.json())
  .then(result => /* process result */)

Response properties:

  • response.status – HTTP code of the response,
  • response.ok – true if the status is 200-299.
  • response.headers – Map-like object with HTTP headers.

Methods to get response body:

  • response.text() – return the response as text,
  • response.json() – parse the response as JSON object,
  • response.arrayBuffer() – return the response as ArrayBuffer (low-level binary data).