Javascript – How to pre-set arguments in JavaScript function call? (Partial Function Application)

functional-programmingjavascript

I am trying to write a JavaScript function that will return its first argument(function) with all the rest of its arguments as preset parameters to that function.

So:

function out(a, b) {
    document.write(a + " " + b);
}

function setter(...) {...}

setter(out, "hello")("world");
setter(out, "hello", "world")();

Would output "hello world" twice. for some implementation of setter

I ran into an issue with manipulating the arguments array on my first try, but it seems there would be a better way to do this.

Best Solution

First of all, you need a partial - there is a difference between a partial and a curry - and here is all you need, without a framework:

function partial(func /*, 0..n args */) {
  var args = Array.prototype.slice.call(arguments, 1);
  return function() {
    var allArguments = args.concat(Array.prototype.slice.call(arguments));
    return func.apply(this, allArguments);
  };
}

Now, using your example, you can do exactly what you are after:

partial(out, "hello")("world");
partial(out, "hello", "world")();

// and here is my own extended example
var sayHelloTo = partial(out, "Hello");
sayHelloTo("World");
sayHelloTo("Alex");

The partial() function could be used to implement, but is not currying. Here is a quote from a blog post on the difference:

Where partial application takes a function and from it builds a function which takes fewer arguments, currying builds functions which take multiple arguments by composition of functions which each take a single argument.

Hope that helps.