Higher-order functions in Javascript

1. Boolean is a function. It’s the function you’re calling indirectly through noisy. A bit confusing, I know, because it looks like the name of a type. But in JavaScript, those initially-capped things (Boolean, Number, String, and so on) are functions. When you call Boolean (without using new), it tries to convert the argument you gave it into a boolean primitive value and returns the result. (See §15.6.1 in the spec.)

2. f is the name of the argument in the noisy function.

Functions in JavaScript are first-class objects. You can pass them into other functions as arguments just like any other object.

When you do

noisy(Boolean)(0)

There are two things going on. First:

// (In effect, we're not really creating a variable...)
var x = noisy(Boolean);

That gives us a function that, when called, will call Boolean with the argument we give it while also doing those console.log statements. This is the function you see being created in noisy (return function(arg)...);

Then we call that function:

x(0);

And that’s when you see the console output. Since Boolean(0) is false, you see Boolean return that value.

Here’s a much simpler example:

function foo(bar) {
    bar();
}
function testing() {
    alert("testing got called");
}
foo(testing);

There, I’m passing the function testing into foo. The argument name I’m using for that within foo is bar. The line bar(); calls the function.