Using Function.prototype.bind with an array of arguments?
functional-programming, javascript, partial-application, promise
Solution
`.bind` is a normal function, so you can call `.apply` on it. All you have to do is pass the original function as the first param and the desired `THIS` variable as the first item in the array of arguments:
bound = db.find.bind.apply(db.find, [null].concat(arguments));
// ^-----^ ^-----^ THIS
Whether that can be considered cleaner or not is left to the reader.
Problem
How can I call Function.prototype.bind with an array of arguments, as opposed to hardcoded arguments? (Not using ECMA6, so no spread operator). I'm trying to put a promises wrapper around a module that uses callbacks and I want to bind all of the arguments passed in to my wrapper method and bind them. Then I want to call the partially applied bound function with my own callback, which will resolve or reject a promise. ``` var find = function() { var deferred, bound; deferred = Q.defer(); bound = db.find.bind(null, arguments); bound(function(err, docs) { if(err) { deferred.fail(err); } else { deferred.resolve(docs); } }); return deferred.promise; } ``` But obviously this doesn't work because bind expects arguments rather than an array of arguments. I know I could do this by inserting my callback onto the end of the arguments array and using apply: ``` arguments[arguments.length] = function(err, docs) { ... } db.find.apply(null, arguments); ``` Or by iterating over the arguments array and rebinding the function for each argument: ``` var bound, context; for(var i = 0; i < arguments.length; i++) { context = bound ? bound : db.find; bound = context.bind(null, arguments[i]); } bound(function(err, docs) { ... }) ``` But both of these methods feel dirty. Any ideas?