Javascript post multipart form data

Sending forms through JavaScript

HTML forms can send an HTTP request declaratively. But forms can also prepare an HTTP request to send via JavaScript, for example via XMLHttpRequest . This article explores such approaches.

A form is not always a form

With progressive web apps, single page apps, and framework-based apps, it’s common to use HTML forms to send data without loading a new document when response data is received. Let’s first talk about why this requires a different approach.

Gaining control of the global interface

Standard HTML form submission, as described in the previous article, loads the URL where the data was sent, which means the browser window navigates with a full page load. Avoiding a full page load can provide a smoother experience by avoiding network lag, and possible visual issues like flickering.

Many modern UIs only use HTML forms to collect input from the user, and not for data submission. When the user tries to send the data, the application takes control and transmits the data asynchronously in the background, updating only the parts of the UI that require changes.

Sending arbitrary data asynchronously is generally called AJAX, which stands for «Asynchronous JavaScript And XML».

How is it different?

The XMLHttpRequest (XHR) DOM object can build HTTP requests, send them, and retrieve their results. Historically, XMLHttpRequest was designed to fetch and send XML as an exchange format, which has since been superseded by JSON. But neither XML nor JSON fits into form data request encoding. Form data ( application/x-www-form-urlencoded ) is made of URL-encoded lists of key/value pairs. For transmitting binary data, the HTTP request is reshaped into multipart/form-data .

Note: The Fetch API is often used in place of XHR these days — it is a modern, updated version of XHR, which works similarly but has some advantages. Most of the XHR code you’ll see in this article could be swapped out for Fetch.

If you control the front-end (the code that’s executed in the browser) and the back-end (the code which is executed on the server), you can send JSON/XML and process them however you want.

But if you want to use a third-party service, you need to send the data in the format the services require.

So how should we send such data? The different techniques you’ll require are done below.

Sending form data

There are 3 ways to send form data:

• Building an XMLHttpRequest manually.
• Using a standalone FormData object.
• Using FormData bound to a element.

Let’s look at them in detail.

Building an XMLHttpRequest manually

XMLHttpRequest is the safest and most reliable way to make HTTP requests. To send form data with XMLHttpRequest , prepare the data by URL-encoding it, and obey the specifics of form data requests.

const btn = document.querySelector("button"); function sendData(data)  console.log("Sending data"); const XHR = new XMLHttpRequest(); const urlEncodedDataPairs = []; // Turn the data object into an array of URL-encoded key/value pairs. for (const [name, value] of Object.entries(data))  urlEncodedDataPairs.push( `$encodeURIComponent(name)>=$encodeURIComponent(value)>`, ); > // Combine the pairs into a single string and replace all %-encoded spaces to // the '+' character; matches the behavior of browser form submissions. const urlEncodedData = urlEncodedDataPairs.join("&").replace(/%20/g, "+"); // Define what happens on successful data submission XHR.addEventListener("load", (event) =>  alert("Yeah! Data sent and response loaded."); >); // Define what happens in case of an error XHR.addEventListener("error", (event) =>  alert("Oops! Something went wrong."); >); // Set up our request XHR.open("POST", "https://example.com/cors.php"); // Add the required HTTP header for form data POST requests XHR.setRequestHeader("Content-Type", "application/x-www-form-urlencoded"); // Finally, send our data. XHR.send(urlEncodedData); > btn.addEventListener("click", () =>  sendData( test: "ok" >); >); 

Note: This use of XMLHttpRequest is subject to the same-origin policy if you want to send data to a third-party website. For cross-origin requests, you’ll need CORS and HTTP access control.

Using XMLHttpRequest and the FormData object

Building an HTTP request by hand can be overwhelming. Fortunately, the XMLHttpRequest specification provides a newer, simpler way to handle form data requests with the FormData object.

The FormData object can be used to build form data for transmission, or to get the data within a form element to manage how it’s sent.

Using this object is detailed in Using FormData Objects, but here are two examples:

Using a standalone FormData object

You should be familiar with that HTML sample. Now for the JavaScript:

const btn = document.querySelector("button"); function sendData(data)  const XHR = new XMLHttpRequest(); const FD = new FormData(); // Push our data into our FormData object for (const [name, value] of Object.entries(data))  FD.append(name, value); > // Define what happens on successful data submission XHR.addEventListener("load", (event) =>  alert("Yeah! Data sent and response loaded."); >); // Define what happens in case of an error XHR.addEventListener("error", (event) =>  alert("Oops! Something went wrong."); >); // Set up our request XHR.open("POST", "https://example.com/cors.php"); // Send our FormData object; HTTP headers are set automatically XHR.send(FD); > btn.addEventListener("click", () =>  sendData( test: "ok" >); >); 

Using FormData bound to a form element

form id="myForm"> label for="myName">Send me your name:label> input id="myName" name="name" value="Dominic" /> input type="submit" value="Send Me!" /> form> 

But JavaScript takes over the form:

.addEventListener("load", () =>  function sendData()  const XHR = new XMLHttpRequest(); // Bind the FormData object and the form element const FD = new FormData(form); // Define what happens on successful data submission XHR.addEventListener("load", (event) =>  alert(event.target.responseText); >); // Define what happens in case of error XHR.addEventListener("error", (event) =>  alert("Oops! Something went wrong."); >); // Set up our request XHR.open("POST", "https://example.com/cors.php"); // The data sent is what the user provided in the form XHR.send(FD); > // Get the form element const form = document.getElementById("myForm"); // Add 'submit' event handler form.addEventListener("submit", (event) =>  event.preventDefault(); sendData(); >); >); 

You can even get more involved with the process by using the form’s elements property to get a list of all of the data elements in the form and manually manage them one at a time. To learn more about that, see the Accessing form controls example.

Dealing with binary data

If you use a FormData object with a form that includes widgets, the data will be processed automatically. But to send binary data by hand, there’s extra work to do.

There are many sources for binary data, including FileReader , Canvas , and WebRTC. Unfortunately, some legacy browsers can’t access binary data or require complicated workarounds. To learn more about the FileReader API, see Using files from web applications.

The least complicated way of sending binary data is by using FormData ‘s append() method, demonstrated above. If you have to do it by hand, it’s trickier.

In the following example, we use the FileReader API to access binary data and then build the multi-part form data request by hand:

form id="theForm"> p> label for="theText">text data:label> input id="theText" name="myText" value="Some text data" type="text" /> p> p> label for="theFile">file data:label> input id="theFile" name="myFile" type="file" /> p> button>Send Me!button> form> 

As you see, the HTML is a standard . There’s nothing magical going on. The «magic» is in the JavaScript:

// Because we want to access DOM nodes, // we initialize our script at page load. window.addEventListener("load", () =>  // These variables are used to store the form data const text = document.getElementById("theText"); const file =  dom: document.getElementById("theFile"), binary: null, >; // Use the FileReader API to access file content const reader = new FileReader(); // Because FileReader is asynchronous, store its // result when it finishes reading the file reader.addEventListener("load", () =>  file.binary = reader.result; >); // At page load, if a file is already selected, read it. if (file.dom.files[0])  reader.readAsBinaryString(file.dom.files[0]); > // If not, read the file once the user selects it. file.dom.addEventListener("change", () =>  if (reader.readyState === FileReader.LOADING)  reader.abort(); > reader.readAsBinaryString(file.dom.files[0]); >); // sendData is our main function function sendData()  // If there is a selected file, wait until it is read // If there is not, delay the execution of the function if (!file.binary && file.dom.files.length > 0)  setTimeout(sendData, 10); return; > // To construct our multipart form data request, // We need an XMLHttpRequest instance const XHR = new XMLHttpRequest(); // We need a separator to define each part of the request const boundary = "blob"; // Store our body request in a string. let data = ""; // So, if the user has selected a file if (file.dom.files[0])  // Start a new part in our body's request data += `--$boundary>\r\n`; // Describe it as form data data += "content-disposition: form-data; " + // Define the name of the form data `name token interpolation">$file.dom.name>"; ` + // Provide the real name of the file `filename token interpolation">$file.dom.files[0].name>"\r\n`; // And the MIME type of the file data += `Content-Type: $file.dom.files[0].type>\r\n`; // There's a blank line between the metadata and the data data += "\r\n"; // Append the binary data to our body's request data += file.binary + "\r\n"; > // Text data is simpler // Start a new part in our body's request data += `--$boundary>\r\n`; // Say it's form data, and name it data += `content-disposition: form-data; name token interpolation">$text.name>"\r\n`; // There's a blank line between the metadata and the data data += "\r\n"; // Append the text data to our body's request data += text.value + "\r\n"; // Once we are done, "close" the body's request data += `--$boundary>--`; // Define what happens on successful data submission XHR.addEventListener("load", (event) =>  alert("Yeah! Data sent and response loaded."); >); // Define what happens in case of an error XHR.addEventListener("error", (event) =>  alert("Oops! Something went wrong."); >); // Set up our request XHR.open("POST", "https://example.com/cors.php"); // Add the required HTTP header to handle a multipart form data POST request XHR.setRequestHeader( "Content-Type", `multipart/form-data; boundary=$boundary>`, ); // Send the data XHR.send(data); > // Get the form element const form = document.getElementById("theForm"); // Add 'submit' event handler form.addEventListener("submit", (event) =>  event.preventDefault(); sendData(); >); >); 

Conclusion

Depending on the browser and the type of data you are dealing with, sending form data through JavaScript can be easy or difficult. The FormData object is generally the answer, and you can use a polyfill for it on legacy browsers.

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