value of variable has changed or not

Question

how to find whether a value of variable is changed or not in javascript .

Answer 1

Ehm?

var testVariable = 10;
var oldVar = testVariable;

...
if (oldVar != testVariable)
alert("testVariable has changed!");

And no, there is no magical "var.hasChanged()" nor "var.modifyDate()" in Javascript unless you code it yourself.

Answer 2

There is a way to watch variables for changes: Object::watch - some code below

/*
For global scope
*/

// won't work if you use the 'var' keyword
x = 10;

window.watch( "x", function( id, oldVal, newVal ){
    alert( id+' changed from '+oldVal+' to '+newVal );

    // you must return the new value or else the assignment will not work
    // you can change the value of newVal if you like
    return newVal;
});

x = 20; //alerts: x changed from 10 to 20


/*
For a local scope (better as always)
*/
var myObj = {}

//you can watch properties that don't exist yet
myObj.watch( 'p', function( id, oldVal, newVal ) {
    alert( 'the property myObj::'+id+' changed from '+oldVal+' to '+newVal );
});


myObj.p = 'hello'; //alerts: the property myObj::p changed from undefined to hello
myObj.p = 'world'; //alerts: the property myObj::p changed from hello to world

// stop watching
myObj.unwatch('p');

Answer 3

Using getters and setters:

var _variable;
get variable() {
    return _variable;
}
set variable(value) {
    _variable = value;
    // variable changed
}

or if you don't need the value of the variable later:

set variable(value) {
    // variable changed
}

Answer 4

If you are a firefox user you can check using firebug. If you are using IE we can put alert statements and check the values of the variables.