How are parameters sent in an HTTP POST request?

Matthew C.

—March 15, 2023

The Problem

You want to send data, using a POST request, to your server that will cause a change on your server. For example, you want to add or modify an item to your database. How do you do that?

The Solution

A POST request is similar to a PUT request, but a PUT request is idempotent. This means that the effect of multiple PUT requests is the same as one PUT request. Two identical PUT requests to add an item to a database will result in one item being added to the database. Two identical POST requests to add an item to a database may result in two items being added to the database.

In a POST request, you pass data in the request body. The type of data in the body is indicated by the Content-Type header of the request.

A POST request is often sent using an HTML form or a fetch request. In a form that has its HTTP method set to “POST”, the content type of the POST request that’s made when the form is submitted is determined by the enctype attribute of the <form> element. The possible values for the enctype are:

• application/x-www-form-urlencoded: This is the default value. The keys and values of the form data are encoded in key-value pairs that are separated by '&', with a '=' between the key and the value. Non-alphanumeric characters are URL encoded. The format is the same as the query strings format in a GET request.

  The following form will send an email and password to the server when the form is submitted:

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  <form action="https://www.example.com/api/auth/login" method="POST">
    <div>
      <label for="email">Email</label>
      <input id="email" name="email" value="" />
    </div>
    <div>
      <label for="password">Password</label>
      <input id="password" type="password" name="password" value="" />
    </div>
    <div>
      <button>Log in</button>
    </div>
  </form>
  

  The POST request will have the data added to the request body:

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  POST / HTTP/2.0
  Host: example.com
  Content-Type: application/x-www-form-urlencoded
  Content-Length: 34
  
  email=bob@example.com&password=123
  

  No data will be appended to the URL. Note that you should never send sensitive data in the query string of a GET request because the data will be displayed in the URL. If you need to send sensitive data or a large amount of data, use a POST request. Some browsers limit the sizes of URLs and many servers limit the length of URLs that they accept.

• multipart/form-data: This is used for sending binary data, like an image file. Each form data value is sent as a block of data (“body part”), with a user agent-defined delimiter (“boundary”) separating each part. The keys are given in the Content-Disposition header of each part. Here’s an example of the body of a POST request that uses the multipart/form-data content type:

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  POST / HTTP/2.0
  Host: example.com
  Content-Type: multipart/form-data; boundary=aBoundaryString
  
  --boundary
  Content-Disposition: form-data; name="name"
  
  (value for name)
  --boundary
  Content-Disposition: form-data; name="profile-pic"; filename="bob.jpg"
  Content-Type: image/jpeg
  
  (value for profile-pic)
  --boundary
  

• text/plain: This is used for debugging.

The value of enctype can be overridden by the formenctype attribute on the <button>, <input type="submit">, or <input type="image"> elements.

When a POST request is sent using a method other than an HTML form, the body can be of any type. For fetch requests, the Content-Type of ‘application/json’ is commonly used to send JSON data to a server:

const login = await fetch('https://www.example.com/api/auth/login', {
  method: 'POST',
  body: JSON.stringify({
    email: 'bob@example.com',
    password: '123',
  }),
  headers: {
    'Content-Type': 'application/json',
  },
});

if (login.ok) {
  console.log('Login successful');
} else {
  throw new Error('Login failed.');
}