Explain "you can have functions that change other functions"

Question

I was reading through Eloquent JavaScript, when I came across this in chapter 5. :

you can have functions that create new functions.

function greaterThan(n) {
  return function(m) { return m > n; };
}
var greaterThan10 = greaterThan(10);

And you can have functions that change other functions.

function noisy(f) {
  return function(arg) {
    console.log("calling with", arg);
    var val = f(arg);
    console.log("called with", arg, "- got", val);
    return val;
  };
}
noisy(Boolean)(0);
//->calling with 0
//->called with 0 - got false

My questions are:

• How are the above two examples different?
• How does noisy change Boolean?

Answer 1

The difference is that the argument to noisy is meant to be another function, rather than a "plain" value like a number. So, yes, it creates a new anonymous function just like greaterThan does, but it's a wrapper around an existing function that modifies its behavior.

In this case, the wrapper just logs some messages before and after calling the original function f. But you could do other things, like modifying its arguments or its return value. For example, you can implement partial function application, which lets you provide some of the arguments for a function call at one point in the program, and "remember" those arguments in a new function that takes just the remaining arguments later.

Answer 2

How are the above two examples different?

greaterThan accepts a parameter, n, which is intended to be a number.

noisy accepts a parameter, f, which is intended to be a function that it can then call within it.

Where greaterThan only evaluates a number, noisy is much more flexible since it can accept any function and execute it.

How does noisy change Boolean?

noisy returns an anonymous function that evaluates Boolean in the line that it stores its results in a variable called val.