How to refactor a promise catch that uses the same function, but also a variable in the calling one?

Just create a function and pass the res object as an argument and return a function.

function makeErrorCatcher(res) {    return function catchesError(err) {        return res.json({            success: false        });    }}router.get('/', function (req, res) {    return somePromise        .then(function doesSomething(someVariable) {            doSomething;        })        .catch(makeErrorCatcher(res));});

You can decorate .get to pass to have a default catch handler. (Assuming you're not interested in a custom router):

Object.keys(router).forEach(function(key){ // for each method    router[key+"P"] = function(path, fn){ // create a decorated alt        router[key].call(router, function(req, res, next){  // delegate            var that = this, args = arguments;            return Promise.try(function(){ // wrap to make throw safe                return fn.apply(that, args); // delegation            }).catch(function catchesError(err){                // LOG YOUR ERRORS, DON'T CATCH ALL                return res.json({ success: false });            });        });    };});

This would let you do:

router.getP('/', function(req, res) {    return somePromise.then(function doesSomething(someVariable) {        doSomething;    });});

Which will now catch errors automatically and send the appropriate JSON. Eliminating duplication or the possibility to forget errors altogether.

Pretty much like Ben Fortune's solution, using bind():

function catchesError(res, err) {    return res.json({ success: false });}router.get('/', function(req, res) {    return somePromise    .then(function doesSomething(someVariable) {        doSomething;    })    .catch(catchesError.bind(null, res));});

Replace null with this if you're in a class.