Inertial: the state that doesn't change instantly. Inertial.set(value, cb, apply) — register intent to change the state to the value. Cb gets called as soon as the state was changed to the value *OR* when the operation failed *OR* when the state did change to another value due to completion of a more recent request. The later requires some clari…

inertial.js

function inertial(v) {
    var self = this;
    self.value = v;
    var q = null;
    self.set = function(value, cb, apply) {
        if (!q && value == v) {
            process.nextTick(function() {
                safeCall(cb, null, value);
            });
        } else {
            if (!q) {
                process.nextTick(function() {
                    apply(cc);
                });
            }
            q = {v:value, apply:apply, cb:cb, next:(q && !q.cb ? q.next : q)};
        }
    }
    function cc(error, value) {
        var i = q;
        if (!error) {
            self.value = v = value;
            while (i && i.v != value) {
                i = i.next;
            }
        } else {
            while (i.next) {
                i = i.next;
            }
        }
        function invoke(i) {
            if (i) {
                invoke(i.next);
                safeCall(i.cb, error, value);
            }
        }
        invoke(i);
        if (i==q) {
            q = null;
        } else {
            var j = q;
            while (j && j.next != i) {
                j = j.next;
            }
            if (j) {
                j.next = null;
            }
            q.apply(cc);
        }
    }
}

function safeCall(fn) {
    if (fn && typeof fn == 'function') {
        fn.apply(this, Array.prototype.slice.call(arguments, 1));
    }
}

output

1 1
!2
2 2
!5
3 5
4 5
5 5

sample.js

test = new inertial(1);
[1,2,3,4,5].forEach(function(v) {
    test.set(v,
        function(error, value) { 
            console.log(v, value);
        },
        function(cb) {
            console.log('!'+v);
            setTimeout(function() {
                cb(null,v);
            }, 1000);
        }
    );
});