JavaScript How to understand composition of functions with multiple arguments?

Such as:

var foo = function (a, b) { return a + b; };
var bar = function (a, b) { return a * b; };

var fn = _.compose([foo, bar]);

How to understand the fn?

Answer:1

This will not work.

First off, _.compose() doesn't take an array. It takes a list of functions as separate arguments as in:

var fn = _.compose(foo, bar);

Then, even with it done that way, with underscore's _.compose(), each successive function is passed the return result of the previous function. Since both your functions require two arguments and return one, this will not work.

If you fix your code to pass the arguments properly to _.compose(), then you'd have this:

var foo = function (a, b) { return a + b; };
var bar = function (a, b) { return a * b; };

var fn = _.compose(foo, bar);

and if you then use fn like this:

fn(2, 5);

it will try to execute:

foo(bar(2, 5));

Breaking this apart, bar(2, 5) will return 10, so you would then have the result of calling foo(10), but that generates NaN because the second argument to foo() is undefined.

_.compose is designed for functions beyond the last one in the list that operate on a single argument that is the return value of the prior function in the list.

Actual implementation showing that it returns NaN: http://jsfiddle.net/jfriend00/xhbcdh9k/

Answer:2

The compose function takes multiple functions and returns a single function. So you would call it as such: var fn = compose(foo, bar);

You can think of fn as nothing more than the result of calling bar then foo. e.g. bar(foo());

Your functions won't compose, since foo expects 2 parameters foo(a, b), but bar returns 1 value.

I wrote about compose, maybe it can help you understand it better.

Answer:3

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