Query parameters are a fundamental part of the HTTP protocol and are extensively utilized in both HTML forms and JavaScript. They're included in the URL following the ? character and are separated by & characters.

In the context of HTML forms, query parameters can be employed to pre-populate form fields. This is achieved by setting the value attribute of the form field to the appropriate query parameter. For instance:

<form action="/search">
  <input type="text" name="q" value="default search term" />
  <input type="submit" value="Search" />
</form>

In this example, the search form is pre-populated with the term "default search term".

JavaScript can read and manipulate query parameters using the URLSearchParams interface. This interface provides methods for working with the query string of a URL. For example:

let params = new URLSearchParams(window.location.search);
let searchTerm = params.get('q');

In this case, JavaScript is used to read the 'q' query parameter from the current URL.

The history.pushState method allows you to manipulate the URL. This enables you to change the URL without triggering a page reload. For instance:

history.pushState({}, '', '?q=new search term');

In this example, the 'q' query parameter is set to "new search term", and the URL is updated without reloading the page. This is a powerful feature that can be used to create a more interactive and user-friendly web experience.