Functions

People think that computer science is the art of geniuses but the actual reality is the opposite, just many people doing things that build on each other, like a wall of mini stones.

Functions are one of the most central tools in JavaScript programming. The concept of wrapping a piece of program in a value has many uses. It gives us a way to structure larger programs, to reduce repetition, to associate names with subprograms, and to isolate these subprograms from each other.

The most obvious application of functions is defining new vocabulary. Creating new words in prose is usually bad style, but in programming, it is indispensable.

Typical adult English speakers have some 20,000 words in their vocabulary. Few programming languages come with 20,000 commands built in. And the vocabulary that is available tends to be more precisely defined, and thus less flexible, than in human language. Therefore, we have to introduce new words to avoid excessive verbosity.

Defining a function

A function definition is a regular binding where the value of the binding is a function. For example, this code defines square to refer to a function that produces the square of a given number:

const square = function(x) {
  return x * x;
};

console.log(square(12));
// → 144

A function is created with an expression that starts with the keyword function. Functions have a set of parameters (in this case, only x) and a body, which contains the statements that are to be executed when the function is called. The body of a function created this way must always be wrapped in braces, even when it consists of only a single statement.

A function can have multiple parameters or no parameters at all. In the following example, makeNoise does not list any parameter names, whereas roundTo (which rounds n to the nearest multiple of step) lists two:

const makeNoise = function() {
  console.log("Pling!");
};

makeNoise();
// → Pling!

const roundTo = function(n, step) {
  let remainder = n % step;
  return n - remainder + (remainder < step / 2 ? 0 : step);
};

console.log(roundTo(23, 10));
// → 20

Some functions, such as roundTo and square, produce a value, and some don’t, such as makeNoise, whose only result is a side effect. A return statement determines the value the function returns. When control comes across such a statement, it immediately jumps out of the current function and gives the returned value to the code that called the function. A return keyword without an expression after it will cause the function to return undefined. Functions that don’t have a return statement at all, such as makeNoise, similarly return undefined.

Parameters to a function behave like regular bindings, but their initial values are given by the caller of the function, not the code in the function itself.

Bindings and scopes

Each binding has a scope, which is the part of the program in which the binding is visible. For bindings defined outside of any function, block, or module (see Chapter 10), the scope is the whole program—you can refer to such bindings wherever you want. These are called global.

Bindings created for function parameters or declared inside a function can be referenced only in that function, so they are known as local bindings. Every time the function is called, new instances of these bindings are created. This provides some isolation between functions—each function call acts in its own little world (its local environment) and can often be understood without knowing a lot about what’s going on in the global environment.

Bindings declared with let and const are in fact local to the block in which they are declared, so if you create one of those inside of a loop, the code before and after the loop cannot “see” it. In pre-2015 JavaScript, only functions created new scopes, so old-style bindings, created with the var keyword, are visible throughout the whole function in which they appear—or throughout the global scope, if they are not in a function.

let x = 10;   // global
if (true) {
  let y = 20; // local to block
  var z = 30; // also global
}

Each scope can “look out” into the scope around it, so x is visible inside the block in the example. The exception is when multiple bindings have the same name—in that case, code can see only the innermost one. For example, when the code inside the halve function refers to n, it is seeing its own n, not the global n.

const halve = function(n) {
  return n / 2;
};

let n = 10;
console.log(halve(100));
// → 50
console.log(n);
// → 10

Nested scope

JavaScript distinguishes not just global and local bindings. Blocks and functions can be created inside other blocks and functions, producing multiple degrees of locality.

For example, this function—which outputs the ingredients needed to make a batch of hummus—has another function inside it:

const hummus = function(factor) {
  const ingredient = function(amount, unit, name) {
    let ingredientAmount = amount * factor;
    if (ingredientAmount > 1) {
      unit += "s";
    }
    console.log(`${ingredientAmount} ${unit} ${name}`);
  };
  ingredient(1, "can", "chickpeas");
  ingredient(0.25, "cup", "tahini");
  ingredient(0.25, "cup", "lemon juice");
  ingredient(1, "clove", "garlic");
  ingredient(2, "tablespoon", "olive oil");
  ingredient(0.5, "teaspoon", "cumin");
};

The code inside the ingredient function can see the factor binding from the outer function, but its local bindings, such as unit or ingredientAmount, are not visible in the outer function.

The set of bindings visible inside a block is determined by the place of that block in the program text. Each local scope can also see all the local scopes that contain it, and all scopes can see the global scope. This approach to binding visibility is called lexical scoping.

Functions as values

A function binding usually simply acts as a name for a specific piece of the program. Such a binding is defined once and never changed. This makes it easy to confuse the function and its name.

But the two are different. A function value can do all the things that other values can do—you can use it in arbitrary expressions, not just call it. It is possible to store a function value in a new binding, pass it as an argument to a function, and so on. Similarly, a binding that holds a function is still just a regular binding and can, if not constant, be assigned a new value, like so:

let launchMissiles = function() {
  missileSystem.launch("now");
};
if (safeMode) {
  launchMissiles = function() {/* do nothing */};
}

In Chapter 5, we’ll discuss the interesting things that we can do by passing function values to other functions.

Declaration notation

There is a slightly shorter way to create a function binding. When the function keyword is used at the start of a statement, it works differently:

function square(x) {
  return x * x;
}

This is a function declaration. The statement defines the binding square and points it at the given function. It is slightly easier to write and doesn’t require a semicolon after the function.

There is one subtlety with this form of function definition.

console.log("The future says:", future());

function future() {
  return "You'll never have flying cars";
}

The preceding code works, even though the function is defined below the code that uses it. Function declarations are not part of the regular top-to-bottom flow of control. They are conceptually moved to the top of their scope and can be used by all the code in that scope. This is sometimes useful because it offers the freedom to order code in a way that seems the clearest, without worrying about having to define all functions before they are used.

Arrow functions

There’s a third notation for functions, which looks very different from the others. Instead of the function keyword, it uses an arrow (=>) made up of an equal sign and a greater-than character (not to be confused with the greater-than-or-equal operator, which is written >=):

const roundTo = (n, step) => {
  let remainder = n % step;
  return n - remainder + (remainder < step / 2 ? 0 : step);
};

The arrow comes after the list of parameters and is followed by the function’s body. It expresses something like “this input (the parameters) produces this result (the body)”.

When there is only one parameter name, you can omit the parentheses around the parameter list. If the body is a single expression rather than a block in braces, that expression will be returned from the function. So, these two definitions of square do the same thing:

const square1 = (x) => { return x * x; };
const square2 = x => x * x;

When an arrow function has no parameters at all, its parameter list is just an empty set of parentheses.

const horn = () => {
  console.log("Toot");
};

There’s no deep reason to have both arrow functions and function expressions in the language. Apart from a minor detail, which we’ll discuss in Chapter 6, they do the same thing. Arrow functions were added in 2015, mostly to make it possible to write small function expressions in a less verbose way. We’ll use them often in Chapter 5.

The call stack

The way control flows through functions is somewhat involved. Let’s take a closer look at it. Here is a simple program that makes a few function calls:

function greet(who) {
  console.log("Hello " + who);
}
greet("Harry");
console.log("Bye");

A run through this program goes roughly like this: the call to greet causes control to jump to the start of that function (line 2). The function calls console.log, which takes control, does its job, and then returns control to line 2. There, it reaches the end of the greet function, so it returns to the place that called it—line 4. The line after that calls console.log again. After that returns, the program reaches its end.

We could show the flow of control schematically like this:

not in function
  in greet
    in console.log
  in greet
not in function
  in console.log
not in function

Because a function has to jump back to the place that called it when it returns, the computer must remember the context from which the call happened. In one case, console.log has to return to the greet function when it is done. In the other case, it returns to the end of the program.

The place where the computer stores this context is the call stack. Every time a function is called, the current context is stored on top of this stack. When a function returns, it removes the top context from the stack and uses that context to continue execution.

Storing this stack requires space in the computer’s memory. When the stack grows too big, the computer will fail with a message like “out of stack space” or “too much recursion”. The following code illustrates this by asking the computer a really hard question that causes an infinite back-and-forth between two functions. Or rather, it would be infinite, if the computer had an infinite stack. As it is, we will run out of space, or “blow the stack”.

function chicken() {
  return egg();
}
function egg() {
  return chicken();
}
console.log(chicken() + " came first.");
// → ??

Optional Arguments

The following code is allowed and executes without any problem:

function square(x) { return x * x; }
console.log(square(4, true, "hedgehog"));
// → 16

We defined square with only one parameter. Yet when we call it with three, the language doesn’t complain. It ignores the extra arguments and computes the square of the first one.

JavaScript is extremely broad-minded about the number of arguments you can pass to a function. If you pass too many, the extra ones are ignored. If you pass too few, the missing parameters are assigned the value undefined.

The downside of this is that it is possible—likely, even—that you’ll accidentally pass the wrong number of arguments to functions. And no one will tell you about it. The upside is that you can use this behavior to allow a function to be called with different numbers of arguments. For example, this minus function tries to imitate the - operator by acting on either one or two arguments:

function minus(a, b) {
  if (b === undefined) return -a;
  else return a - b;
}

console.log(minus(10));
// → -10
console.log(minus(10, 5));
// → 5

If you write an = operator after a parameter, followed by an expression, the value of that expression will replace the argument when it is not given. For example, this version of roundTo makes its second argument optional. If you don’t provide it or pass the value undefined, it will default to one:

function roundTo(n, step = 1) {
  let remainder = n % step;
  return n - remainder + (remainder < step / 2 ? 0 : step);
};

console.log(roundTo(4.5));
// → 5
console.log(roundTo(4.5, 2));
// → 4

The next chapter will introduce a way in which a function body can get at the whole list of arguments it was passed. This is helpful because it allows a function to accept any number of arguments. For example, console.log does this, outputting all the values it is given:

console.log("C", "O", 2);
// → C O 2

Closure

The ability to treat functions as values, combined with the fact that local bindings are re-created every time a function is called, brings up an interesting question: What happens to local bindings when the function call that created them is no longer active?

The following code shows an example of this. It defines a function, wrapValue, that creates a local binding. It then returns a function that accesses and returns this local binding.

function wrapValue(n) {
  let local = n;
  return () => local;
}

let wrap1 = wrapValue(1);
let wrap2 = wrapValue(2);
console.log(wrap1());
// → 1
console.log(wrap2());
// → 2

This is allowed and works as you’d hope—both instances of the binding can still be accessed. This situation is a good demonstration of the fact that local bindings are created anew for every call, and different calls don’t affect each other’s local bindings.

This feature—being able to reference a specific instance of a local binding in an enclosing scope—is called closure. A function that references bindings from local scopes around it is called a closure. This behavior not only frees you from having to worry about the lifetimes of bindings but also makes it possible to use function values in some creative ways.

With a slight change, we can turn the previous example into a way to create functions that multiply by an arbitrary amount.

function multiplier(factor) {
  return number => number * factor;
}

let twice = multiplier(2);
console.log(twice(5));
// → 10

The explicit local binding from the wrapValue example isn’t really needed since a parameter is itself a local binding.

Thinking about programs like this takes some practice. A good mental model is to think of function values as containing both the code in their body and the environment in which they are created. When called, the function body sees the environment in which it was created, not the environment in which it is called.

In the previous example, multiplier is called and creates an environment in which its factor parameter is bound to 2. The function value it returns, which is stored in twice, remembers this environment so that when that is called, it multiplies its argument by 2.

Recursion

It is perfectly okay for a function to call itself, as long as it doesn’t do it so often that it overflows the stack. A function that calls itself is called recursive. Recursion allows some functions to be written in a different style. Take, for example, this power function, which does the same as the