Question

If I have a variable in an outer function, and an internal function that changes that variable, why does that variable revert back to its original value?

// Check which type of event registration browser supports
function addEvent(eventTarget, eventType, eventHandler){
    if (window.addEventListener){
        eventTarget.addEventListener(eventType, eventHandler);
    }
    else if (window.attachEvent){
        var eventType = "on" + eventType;
        eventTarget.attachEvent(eventType, eventHandler);
    }
}

// Check if browser supports DOMContentLoaded
var DOMContentLoadedSupported = "no";

addEvent(document, "DOMContentLoaded", function(){
    DOMContentLoadedSupported = "yes";
})

alert(DOMContentLoadedSupported) // alerts "no" not "yes". Why??            
​

Answer 1

The function will modify the variable, but that will happen AFTER your alert, so you see “No” in the alert cause the alert happens before the variable gets modified.