Functions are Values

Functions are values in JavaScript. Just like 5 is a value that you can assign to a variable, the function that returns its input plus one is a value that you can assign to the variable addOne.

Because functions are values, you can do lots of cool things with them. You can put them in a list.

var f = function (x) { return x + 1; }; var g = function(x) { return x * 2; }; var lst = [f, g, f]; // a list of functions! var y = lst[1](5); // same as g(5)

Because functions are values, you can pass functions as arguments and return them from other functions. Here is a definition of callTwice that takes a function f as an argument and invokes it twice in a row.

var addOne = function (x) { return x + 1; }; var callTwice = function(f, x) { return f(f(x)); }; var y = callTwice(addOne, 3); // => addOne(addOne(3)) => 5

Define a function named applyTest that takes two arguments, a function f and an input x. Your function should return a list that contains x, the function f on the input x, and the function f on the input 0.