How to alert after paste event in Javascript?

You could put your alert in a setTimeout.

setTimeout(function() {alert('Pasted');}, 0);

This will delay the code until after the value has updated.

Just keep in mind that this in the setTimeout callback will have a different value than that in the enclosing environment.

If you'll need a reference to the outer this, which will be the element, then reference it in a variable...

var self = this;
setTimeout(function() {alert(self.value);}, 0);

Or use .bind() (Supported in most browsers that support addEventListener. Older Safari didn't support it.)...

setTimeout(function() {alert(this.value);}.bind(this), 0);

setTimeout seems the best option and you can use something like this to generalise

// object definition

    function PasteMonitor(element)
    {
        if(typeof element == "string")
        {
            this.target = document.getElementById(element);
        }
        else if(typeof element == "object" || typeof element == "function")
        {
            this.target = element;
        }

        if(this.target != null && this.target != undefined)
        {
            this.target.addEventListener('paste',this.inPaste.bind(this),false);
            this.target.addEventListener('change',this.changed.bind(this),false);
        }
        this.oldstate = "";
    }
    PasteMonitor.prototype = Object.create({},{
        pasted:{ value: false, enumerable: true, configurable: true, writable: true },
        changed:{ value: function(evt){
            //elements content is changed
            if(typeof this.onChange == "function")
            {
                this.onChange(evt);
            }
            if(this.pasted)
            {
                if(typeof this.afterPaste == "function")
                {
                    this.afterPaste(evt);
                    this.pasted = false;
                }
            }
        }, enumerable: true, configurable: true, writable: true },
        inPaste:{ value: function(evt){
            var cancelPaste = false;
            if(typeof this.beforePaste == "function")
            {
                // process pasted data
                cancelPaste = this.beforePaste(evt);
            }
            if(cancelPaste == true)
            {
                evt.preventDefault();
                return false;
            }
            this.pasted = true;
            setTimeout(function(){
                var evt = document.createEvent("HTMLEvents");
                evt.initEvent("change", false, true);
                this.target.dispatchEvent(evt);
            }.bind(this),0);
        }, enumerable: true, configurable: true, writable: true }
    })
    PasteMonitor.prototype.constructor = PasteMonitor;

//PasteMonitor usage

    //var element = document.getElementById('paste_area');
    var msgArea = document.getElementById('message');
    var myMonitor = new PasteMonitor("paste_area");
    //or use and object as argument => var myMonitor = new PasteMonitor(element);

    myMonitor.onChange = function(evt){
        if(this.pasted)
        {
            //do something on paste change
            msgArea.innerHTML = "onChange by paste";
            this.oldstate = this.target.value;
        }
        //check to prevent processing change event when the element loses focus if the change is done by paste
        else if(this.target.value != this.oldstate)
        {
            msgArea.innerHTML = "onChange by typing";
        }
    };
    myMonitor.afterPaste = function(evt){
       // do something after paste
        msgArea.innerHTML = "afterPaste";
    };
    myMonitor.beforePaste = function(evt){
        // do something before the actual paste
        msgArea.innerHTML = "beforePaste";
        //return true to prevent paste
        return false;
    };