JavaScript: Function as arguments

Wanted to create a chain of method call backs with some simple JavaScript as in

startMethod -> calls function on success and another function on failure and it had been awhile since I did this refreshed my memory on Promises and Object.assign

First create a variable like so to hold both branches of execution like so

var actions = {
	onSuccessFn: function() {
    	console.log("All good");
    onFailureFn: function() {
    	console.log("No no no");


Define the start method to handle the override functions as args

function startMethod(overrideFns) {
    // Assign defaults and fallback to null if none given

	var currentCallbacks = Object.assign({
    	onSuccessFn: null,
        onFailureFn: null
	}, overrideFns || {});

    // Promise currentPromise gets defined here    

    currentPromise.then(() => {
        if ((currentCallbacks.onSuccessFn !== null) &&
            (typeof currentCallbacks.onSuccessFn === "function")) {
        } else {
    .catch(err => {
        if ((currentCallbacks.onFailureFn !== null) &&
            (typeof currentCallbacks.onFailureFn === "function")) {
        } else {

This is close enough to what I wrote to trigger my memory, so need this to jog my memory on promises.  Was debating on using the shortened method of == relying on JavaScript implicit conversion like so

currentCallbacks.onSuccessFn instanceof Function but I am not as comfortable with this and decided to stick with the more explicit null check and function type comparison