如何在javascript中定义全局变量

Question

I am wondering how I can define a global variable in javascript. That's what I am doing and I want currentValue to be global so everytime gotoNext and previous function can change its value

onPageClick : function(event) {
                var currentValue = event.target.innerHTML;

                if (currentValue == "Next")
                    this.gotoNext();

                if (currentValue == "previous")

                    this.gotoPrev();

            },

gotoNext : function() {
                this.currentValue +1;

            },

gotoPrevious : function() {
                this.currentValue -1;

            },

but currentValue is not defined in goToNext and gotoPrevious!

Answer 1

You're not really asking about a global variable, but a class level variable.

You need to set this.currentValue = event.target.innerHTML;

Answer 2

You need to define currentValue outside of any function scope. Like this:

<script>
var currentValue = '';

/* YOUR SCRIPT GOES HERE */

</script>

But then, you must change your code on the two last functions:

gotoNext : function() {
                currentValue += 1;

            },

gotoPrevious : function() {
                currentValue -= 1;

            }

But be carefull with this, globale variables are a bad thing. IMO, the best way to define an object is as follow:

var yourObject = function(){
    var currentValue = '';

    var that = {
            onPageClick : function(event) {
                var currentValue = event.target.innerHTML;

                if (currentValue == "Next")
                    that.gotoNext();

                if (currentValue == "previous")

                    that.gotoPrevious();

            },

            gotoNext : function() {
                currentValue += 1;

            },

           gotoPrevious : function() {
                currentValue -= 1;

            }
       };

       return that;

};

And then, you use it like this:

var obj = yourObject();
obj.gotoNext();
obj.gotoPrevious();

I would advise you to learn about scope and closures :

http://robertnyman.com/2008/10/09/explaining-javascript-scope-and-closures/