Stream.js

function foreach(a, f) {
    var total,
        i;
 
    for (i = 0, total = a.length; i < total; i++) {
        f(a[i]);
    }
}
 
function Propagate(value){
    this.value = value;
}
function Negate() {
}
 
function Stream(pulse) {
    var siblings = [];
    
    pulse(function(value){
        for(var i = 0, total = siblings.length; i < total; i++) {
            siblings[i](value);
        }
    });
    
    this.fork = function(func) {
        var binder;
        siblings.push(function pulser(value) {
            var propagation = func(value);
            if (binder && propagation instanceof Propagate) {
                binder(propagation.value);
            } else {
                binder = null;
                // This could be done in a FP way.
                var index = siblings.indexOf(pulser);
                if (index >= 0) {
                    siblings.splice(index, 1);
                }
            }
        });
        
        return new Stream(function(pulse) {
            binder = pulse;
        });
    };
}
 
Stream.prototype.foreach = function(f) {
    return this.fork(f);
};
 
Stream.prototype.map = function(f) {
    return this.fork(function(value) {
        return new Propagate(f(value));
    });
};
 
function StreamBool(stream){
    this.stream = stream;
}
 
StreamBool.prototype.not = function() {
    return new StreamBool(this.stream.map(function(value) {
        return !value;
    }));
};
 
var stream = new Stream(function(pulse) {
    setTimeout(function() {
        pulse((Math.random() * 2) - 1);
    }, 100);
});
 
stream.fork(function(value) {
    console.log('Foreach 1 A', value);
    return new Propagate(value);
}).foreach(function(value) {
    console.log('Foreach 2 A', value);
    return new Negate();
}).foreach(function(value) {
    console.log('Foreach 3 A', value);
});

stream.fork(function(value) {
    console.log('Foreach 1 B', value);
    return new Propagate(value);
});

stream.map(function(value) {
    console.log('Map 1', value);
    return value + 1;
}).map(function(value) {
    console.log('Map 2', value);
    return value + 1;
});

 
var bool = new StreamBool(stream);
bool.not().stream.foreach(function(value) {
    console.log('Not 1', value);
});

Outputs:
// Foreach 1 A, 0.8283838094212115
// Foreach 2 A, 0.8283838094212115
// Foreach 1 B, 0.8283838094212115
// Map 1, 0.8283838094212115
// Map 2, 1.8283838094212115
// Not 1, false