A suggested utility function based on RxJS

gate.js

// Gate takes three Observables.
// gateFlag$: an Observable of booleans
// open$:     an Observable of values
// closed$:   an Observable of values

var gate = function(gateFlag$, open$, closed$) { 
  ... 
};

// Use Case: 
// I have an observable of booleans from a checkbox event in the browser. 
// Using the `gate` function to combine the streams : `var gated$ = gate(checkbox$, a$, b$)`
// To update the state of gated$, it should apply the following rules: 

// When the checkbox *becomes* checked, I want to push the latest result of observable A
// When the checkbox *becomes* unchecked, I want to push the latest result of observable B

// When the checkbox is already checked, and a new value is pushed to observable A, I want to push that value
// When the checkbox is already unchecked, and a new value is pushed to observable B, I want to push that value