Post Form Data With Javascript Fetch (Very Simple Example)

Welcome to a quick tutorial and example on how to post form data using Javascript Fetch. So you have just started with the Javascript Fetch API, and wondering how to send data with it?

That covers the quick basics, but read on for a detailed example!

ⓘ I have included a zip file with all the source code at the start of this tutorial, so you don’t have to copy-paste everything… Or if you just want to dive straight in.

 

 

TLDR – QUICK SLIDES

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Fullscreen Mode – Click Here

 

TABLE OF CONTENTS

 

 

 

PART 1) HTML FORM

<form onsubmit="return sendData()" id="demo">
  <input type="text" name="name" value="Jon Doe" required>
  <input type="email" name="email" value="jon@doe.com" required/>
  <input type="submit" value="Go!">
</form>

For this simple example, we have a dummy HTML form with only 2 fields – Name and email.

 

PART 2) JAVASCRIPT FETCH 

function sendData () {
  // (A) GET FORM DATA
  var data = new FormData(document.getElementById("demo"));
  // data.append("KEY", "VALUE");
 
  // (B) INIT FETCH POST
  fetch("2-dummy.php", {
    method: "POST",
    body: data
  })
 
  // (C) RETURN SERVER RESPONSE AS TEXT
  .then(res => {
    if (res.status != 200) { throw new Error("Bad Server Response"); }
    return res.text();
  })
 
  // (D) SERVER RESPONSE
  .then(res => console.log(res))
 
  // (E) HANDLE ERRORS - OPTIONAL
  .catch(err => console.error(err));
 
  // (F) PREVENT FORM SUBMIT
  return false;
}

This is pretty much the “full version” of the introduction snippet, should be pretty self-explanatory:

1. Create a FormData() object, collect all the data that needs to be sent to the server.
   • var data = new FormData(HTML-FORM) will automatically adapt all form fields name="value" into data.
   • We can manually add more entries with data.append("KEY", "VALUE").
   • Also, use data.delete("KEY") to remove entries if required.
2. As in the introduction, set fetch to method: "POST" and append the form data body: data.
3. Return the server response as a string. Take note of if (res.status != 200) { THROW ERROR } here. For you guys who are new, fetch will consider it a “success” as long as the server responds – Even when the server responds with errors like 404 (not found), 403 (unauthorized), and 500 (internal server error). So, it is safer to do a manual 200 (OK) check here.
4. Do something with the server response. Use this to show a success/failure message to the user alert(res), or put it into an HTML element document.getElementById("ID").innerHTML = res.
5. Handle any errors. Optional, but highly recommended.

 

 

PART 3) SERVER-SIDE

<?php
print_r($_POST);

Lastly, do processing on the server-side as required – This dummy PHP script simply echos whatever is being posted over.

 

 

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EXAMPLE CODE DOWNLOAD

Click here for the source code on GitHub gist, just click on “download zip” or do a git clone. I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

COMPATIBILITY CHECK

• Fetch API – CanIUse
• Form Data – CanIUse

Fetch is already well-supported in all modern browsers. Just take extra note if you have to support the ancient Internet Exploders.

 

LINKS & REFERENCES

• Fetch API – MDN
• HTTP Response Codes – Wikipedia

 

 

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