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How to call server side function from html button (not in a form) using ExpressJS

I have an express app with an index.js that has the following:

<form method="post" action="searchAll">
    <input type="text" name="keyword"/>
    <button type="submit">Submit</button>
</form>

This functions well as a form that takes a keyword and searches my database and then follows up with a POST of the results.

But can I have a button like this:

<button id="refreshDB">REFRESH DATABASE</button></br>

which doesn't send any data to the server other than calling a server side function? The function (located in app.js or db.js on the server) takes no parameters and doesn't follow up with a post request. I'm thinking something like the following:

<button id="refreshDB">REFRESH DATABASE</button>
<script>
    var button = document.getElementById('refreshDB');
    button.addEventListener('click', function() {

    // SOMEHOW TELL SERVER TO RUN FUNCTION
    // similar to the html <form method="post" action="refreshDB">?

    });
</script>

I know I'm missing something really basic. I have a basic understanding of routing, but have no idea how to use one for a simple one-way function call. All of the help I find usually uses a form to submit data. The ExpressJS documentation is helpful, but I can only find server side code when it comes to routes like this.

This popular question is asking something similar, but the answer uses a form.

Can help identify what basic 'thing' I am missing? Thank you!

about 4 years ago · Santiago Trujillo

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Accepted Answer · 2022-06-14T18:28:08.157Z

You need to do an AJAX call to your server (an API route) and then deal with the result.

An 'AJAX call' is an asynchronous HTTP request, that means that you won't have to reload your page to get the response of your request (unlike the form tag).

Make an AJAX call

Javascript (pure / Vanilla) [ON THE CLIENT SIDE]

function myAjaxCall(url, data, callback) {
  var req = new XMLHttpRequest();
  req.open("POST", url, true);
  req.send(data);
  req.onload = function(e) {
    if (this.status == 200) { // if the HTTP response code is 200 (OK)
      callback(e.responseText); // passing the result of the request to the callback function 
    }
  };
}

JQuery [ON THE CLIENT SIDE]

$.ajax({
  method: "POST",
  url: "http://your.server/route/url",
  data: "The data you want to send to your server"
}).done(function(res) {
  console.log(res); // the value returned by the server (deal with it)
});

EDIT : Making it work with the button [ON THE CLIENT SIDE]

var yourData = "The data you want to send to your server";
var url = "http://your.server/route/url";

var button = document.getElementById('refreshDB');
button.onclick = function(e) { // (almost the) same as addEventListener
  myAjaxCall(url, yourData, function uselessName(result) {
    console.log(result);
  });
}

Creating an API route with Express and Nodejs [ON THE SERVER SIDE]

I assume that you already have an Express server, so you have the app object.
Let's say that your server address is 'http://your.server'. The simpliest way to create a POST route (not the better if you're building a big app) is the following :

app.post('/route/url', function(req, res) {
  console.log(req.body); // this will output "The data you want to send to your server"
  var data = yourFunction();
  res.status(200).send(data); // the status 200 is the default one, but this is how you can simply change it
})

EDIT - here's what actually happen :

You click on the button, it enters into the event handler (the function after the onclick keyword)
It uses the function myAjaxCall
This function makes the request (Ajax call)
req.onload is called (because it recieve the response of the request, that mean the data coming from the server)
we use the function that is passed in parameter as 3rd argument of the myAjaxCall function and pass the result of the request as a parameter
this function (named uselessName because in reality it doesn't need a name) will just log the result.

Hope it helps,

Best regards

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