How to call a REST web service API from JavaScript?

Your Javascript:

function UserAction() {
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
         if (this.readyState == 4 && this.status == 200) {
             alert(this.responseText);
         }
    };
    xhttp.open("POST", "Your Rest URL Here", true);
    xhttp.setRequestHeader("Content-type", "application/json");
    xhttp.send("Your JSON Data Here");
}

Your Button action::

<button type="submit" onclick="UserAction()">Search</button>

For more info go through the following link (Updated 2017/01/11)

I'm surprised nobody has mentioned the new Fetch API, supported by all browsers except IE11 at the time of writing. It simplifies the XMLHttpRequest syntax you see in many of the other examples.

The API includes a lot more, but start with the fetch() method. It takes two arguments:

1. A URL or an object representing the request.
2. Optional init object containing the method, headers, body etc.

Simple GET:

const userAction = async () => {
  const response = await fetch('http://example.com/movies.json');
  const myJson = await response.json(); //extract JSON from the http response
  // do something with myJson
}

Recreating the previous top answer, a POST:

const userAction = async () => {
  const response = await fetch('http://example.com/movies.json', {
    method: 'POST',
    body: myBody, // string or object
    headers: {
      'Content-Type': 'application/json'
    }
  });
  const myJson = await response.json(); //extract JSON from the http response
  // do something with myJson
}