Calling a javascript function without arguments

Question

Is it possible to call a javascript function without paranthesis? (). In the below code, from a book, has the line,

http.onreadystatechange = useHttpResponse;

If there is no paramenters in the function definition, can we call without arguments?

function getServerText() {
  var myurl = 'ajax.php';
  myRand = parseInt(Math.random() * 999999999999999);
  var modurl = myurl + "?rand=" + myRand;
  http.open("GET", modurl, true);
  http.onreadystatechange = useHttpResponse;
  http.send(null);
}


function useHttpResponse() {
  if (http.readyState == 4) {
    if (http.status == 200) {
      var mytext = http.responseText;
      document.getElementById('myPageElement')
        .innerHTML = mytext;
    }
  } else {
    document.getElementById('myPageElement')
      .innerHTML = "";
  }
}

The only situation where you can omit parenthesis is when using the new keyword. Or you use something hacky like setTimeout(func, 0), but that's not really reasonable IMO. edit: As Niet said, in this case you are not calling the function and you don't want to. You are assigning a function as callback so that other code can call the function later. — Felix Kling
– Felix Kling, Commented May 26, 2014 at 21:42
Here http.onreadystatechange = useHttpResponse is a reference to useHttpResponse function. You can do it partly because functions are first class objects in JavaScript. You can assign a function to a variable: var square = function(x) { return x*x; }. You can even have an array of functions: var fs = [square, function(x) { return x*x*x; }] You might find it interesting that yes you can invoke a function with a syntax like this: square.apply(null, [5]). Notice how I'm calling apply function on square function. — homam
– homam, Commented May 26, 2014 at 21:50

Niet the Dark Absol · Accepted Answer · 2014-05-26 21:42:50Z

3

When assigning an event handler, you don't want to call the function, you're giving it a reference to a function, which will be called later.

So... no. () is used to mean "call this function", in this case with no arguments.

answered May 26, 2014 at 21:42

Niet the Dark Absol

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Comments

Mister Jojo · Accepted Answer · 2021-09-26 11:57:53Z

0

It is the same way of :

function add2values(a, b) {
  return a + b
}

const objX = { name: 'xyz', myAdd: null }

objX.myAdd = add2values

console.log( objX.myAdd(1, 2) ) // -> 3

answered Sep 26, 2021 at 11:57

Mister Jojo

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