Calling a JavaScript function

Question

To call, for example, a function named makeRequest, I learned you need to do makeRequest();. If you only do makeRequest; it is a reference to the function (I thought). Yet, looking at this code (which worked when I tested it), it calls makeRequest; on the window.onload without the parens.

Can someone explain?

window.onload = makeRequest;
var xhr = false;

function makeRequest() {
    if (window.XMLHttpRequest) {
        xhr = new XMLHttpRequest();
    }
    else {
        if (window.ActiveXObject) {
            try {
                xhr = new ActiveXObject("Microsoft.XMLHTTP");
            }
            catch (e) { }
        }
    }

    if (xhr) {
        xhr.onreadystatechange = showContents;
        xhr.open("GET", "colors.xml", true);
        xhr.send(null);
    }
    else {
        document.getElementById("updateArea").innerHTML = "Sorry, but I couldn't create an XMLHttpRequest";
    }
}

function showContents() {
    if (xhr.readyState == 4) {
        if (xhr.status == 200) {
            var outMsg = xhr.responseText;
        }
        else {
            var outMsg = "There was a problem with the request " + xhr.status;
        }
        document.getElementById("updateArea").innerHTML = outMsg;
    }
}

Darin Dimitrov · Accepted Answer · 2011-04-08 06:51:35Z

6

On this line:

window.onload = makeRequest;

the function makeRequest is not called. You only assign the function pointer to the onload event. When the DOM is loaded the browser automatically calls this event and it it is only at that moment that the function it is pointing to is invoked (which might happen much later but leave you with the impression that the function is called immediately).

answered Apr 8, 2011 at 6:51

Darin Dimitrov

1.0m275 gold badges3.3k silver badges3k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Bruce · Accepted Answer · 2011-04-08 06:52:12Z

3

It's a reference when used on the right-side of an expression. ie

var x = myfunction;

would set x to be a "pointer" to myfunction.

var x = myfunction();

would store the value returned-by-myfunction into x.

answered Apr 8, 2011 at 6:52

Bruce

9113 gold badges7 silver badges12 bronze badges

Collectives™ on Stack Overflow

Calling a JavaScript function

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related