Can I detect if a variable in javascript increases or decreases in value?

Question

I have a a variable, which always is an integer, that increases and/or decreases in value from time to time, goes from 1-5. I wanted to know if I can, in any way, detect if it goes up or down, for example:

The variable is currently at three, if it increases to four, it runs a function, but if it decreases to two, it runs another function.

Can that be done?

Answer 1

The following four methods will work depending on support by browser compatibility, and this post did take me a while, but it also taught me what ___ does, and I hope I listed most methods, and if you know more, you could post an answer. However, it would be greatful of you to post your method in the comments so I can add it to this answer. Cheers!

---

1 Use Object.observe:

Object.observe(i, function(b){ b.object[change.name] > b.type ? console.log('decreased') : console.log('increased'); });

Additional notes about Object.observe

If you like to enable it in Chrome 33,

Visit chrome://flags/

And enable Enable Experimental JavaScript

---

2 Use setInterval:

var i = 4;
var a = i;
setInterval(function()
{
    if(i != a) a > i ? console.log('decreased') : console.log('increased');
}, 1000);
i = 10;

should log 'increase' in one second.

---

3 Use Object.watch:

var i = {i:5};
i.watch("i", function (id, oldval, newval) {
    newval > oldval ? console.log('decreased') : console.log('increased');
    return newval;
});

Additional notes about Object.watch

To support more browsers, add this script:

if (!Object.prototype.watch) {
    Object.defineProperty(Object.prototype, "watch", {
          enumerable: false
        , configurable: true
        , writable: false
        , value: function (prop, handler) {
            var
              oldval = this[prop]
            , newval = oldval
            , getter = function () {
                return newval;
            }
            , setter = function (val) {
                oldval = newval;
                return newval = handler.call(this, prop, oldval, val);
            }
            ;

            if (delete this[prop]) { // can't watch constants
                Object.defineProperty(this, prop, {
                      get: getter
                    , set: setter
                    , enumerable: true
                    , configurable: true
                });
            }
        }
    });
}

// object.unwatch
if (!Object.prototype.unwatch) {
    Object.defineProperty(Object.prototype, "unwatch", {
          enumerable: false
        , configurable: true
        , writable: false
        , value: function (prop) {
            var val = this[prop];
            delete this[prop]; // remove accessors
            this[prop] = val;
        }
    });
}

obtained from https://gist.github.com/eligrey/384583

---

4. Use a custom value set function:

i = 4;

function setValue(obj, val)
{
    val < obj ? console.log('decrease') : console.log('increase');
    obj = val;
}

setValue(i, 10);

should log 'increase'.

---

Most of these require the variable to be an Object, so I'd rather detect over time as seen in method 2.

Answer 2

let prev;

const change = (i) => {

    if (prev === undefined) prev = i;

    const next = i;

    if (prev < next) console.log("increase");

    if (prev > next) console.log("decrease");

    prev = next;
};