JavaScript partially applied function - How to bind only the 2nd parameter?

Question

Sorry if I\'m missing something obvious, but I can\'t figure out how to bind a specific (nth) argument of a function in javascript. Most of my functional programming I\'ve

Answer 1

Of course you can do it. Here's an ES6 solution using the spread operator (...), since it's a bit more compact.

// Bind arguments starting after however many are passed in.
function bind_trailing_args(fn, ...bound_args) {
    return function(...args) {
        return fn(...args, ...bound_args);
    };
}

If you'd prefer to specify the position at which binding starts:

// Bind arguments starting with argument number "n".
function bind_args_from_n(fn, n, ...bound_args) {
    return function(...args) {
        return fn(...args.slice(0, n-1), ...bound_args);
    };
}

IN ES5, you have to muck around with constructing argument lists.

// ES5 version: construct arguments lists yourself
function bind_trailing_args(fn) {
    var bound_args = [].slice.call(arguments, 1);
    return function() {
        var args = [].concat.call(arguments, bound_args);
        return fn.apply(this, args);
    };
}

Unlike the first two examples, this one handles this properly.

In the context of your example:

var addThree = bind_trailing_args(add, 3);
addThree(1) // calls add(1, 3)

You could also consider using one of the functional programming libraries available for JS, such as http://osteele.com/sources/javascript/functional/. The thing you want is called rcurry there.