Learning about functional programming, inheritance, etc

Frontend Masters - JS the good parts

// Write a function that takes an argument and
// returns that argument.

function identity(id) {
	return id;
}

// Write two binary functions, 'add' and 'mul',
// that take two numbers and return their sum and product.

function add(x, y) {
	return x + y;
}

function mul(x, y) {
	return x * y;
}

// Write a function that takes an argument and returns
// a function that returns that argument.

function identityf(x) {
	return function() {
		return x;
	};
}

// Write a function that adds from two invocations

function addf(x) {
	return function(y) {
		return x + y;
	};
}

// Write a function that takes a binary function, and
// makes it callable with two invocations.

function applyf(binFunc) {
	return function (x) {
		return function (y) {
			return binary(x, y);
		};
	};
}

// Write a function that takes a function and an argument,
// and returns a function that can supply a second argument.

function curry(binary, x) {
	return function (y) {
		return binary(x, y);
	};
}

// Without without writing any new functions, 
// show three ways to create the 'inc' function.
// inc(5) 		// 6
// inc(inc(5))	// 7

inc = addf(1);
inc = applyf(add)(1);
inc = curry(add, 1);

// ****** I Don't know how using these three can make inc(5) into 6.
// In addf, for example, how is the function getting the y value? is 
// the y value what the value of inc already is?