onClick going off without being clicked

Question

I'm very new to writing with JavaScript and am creating a basic tab navigation interface as a prototype/practice. Right now I'm trying to make an alert appear using the onclick event handler on an unordered list of links just as a test and can't figure it out. Every time I load the page, all 5 onclick events occur without having clicked anything. Take a look at the code and let me know if you have any suggestions:

window.onload = initAll;
var xhr = false;

function initAll () {
    var navDiv = document.getElementById("tab_navigation");
    var navTabs = navDiv.getElementsByTagName("a");

    for (var i = 0; i < navTabs.length; i++) {
        navTabs[i].onclick = tellTab(i);
    }
}


function tellTab (number) {
    number = number + 1;
    alert("You clicked tab number " + number);
}

The HTML structure is as follows:

    <div id="tab_navigation">
        <ul>
            <li><a href="#1">Tab 1</a></li>
            <li><a href="#2">Tab 2</a></li>
            <li><a href="#3">Tab 3</a></li>
            <li><a href="#4">Tab 4</a></li>
            <li><a href="#5">Tab 5</a></li>
        </ul>
    </div>   

Answer 1

In a for loop, you have this line:

navTabs[i].onclick = tellTab(i);

That calls tellTab immediately. What you can do is have tellTab return a closure, like this:

function tellTab (number) {
    return function() {
        // I commented out the following line
        // because I don't think it's necessary.
        //number = number + 1; 
        alert("You clicked tab number " + number);
    };
}

A different option is creating a closure inside the for loop:

for (var i = 0; i < navTabs.length; i++) {
    (function(i) {
        navTabs[i].onclick = function() {
            tellTab(i);
        };
    })(i);
}

Answer 2

Closures for dummies:

function initAll () {
    var navDiv = document.getElementById("tab_navigation");
    var navTabs = navDiv.getElementsByTagName("a");

    for (var i = 0; i < navTabs.length; i++) {
        setTabOnclick(navTabs[i], i);
    }
}

function setTabOnclick(tab, i) {
   tab.onclick = function () { tellTab(i); };
}

function tellTab(i) {
  alert('clicked tab ' + i);
}

I've just realized that this is the same as icktoofay's second solution but with a named outer function.