JavaScript Functions

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". -- Donald Knuth

Functions are the bread and butter of 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 usually have to introduce new concepts to avoid repeating ourselves too much.

Source: Marijn Haverbeke, https://eloquentjavascript.net/03_functions.html
This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 License.

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 function 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 power lists two:

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

makeNoise();
// → Pling!

const power = function(base, exponent) {
  let result = 1;
  for (let count = 0; count < exponent; count++) {
    result *= base;
  }
  return result;
};

console.log(power(2, 10));
// → 1024

Some functions produce a value, such as power and square, 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 or block, the scope is the whole program - you can refer to such bindings wherever you want. These are called global.

But 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 that they are declared in, 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 that they appear in - or throughout the global scope, if they are not in a function.

let x = 10;
if (true) {
  let y = 20;
  var z = 30;
  console.log(x + y + z);
  // → 60
}
// y is not visible here
console.log(x + z);
// → 40

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.

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, which is 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. 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 pass to a function. If you pass too many, the extra ones are ignored. If you pass too few, the missing parameters get 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 this behavior can be used 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 power makes its second argument optional. If you don't provide it or pass the value undefined, it will default to two, and the function will behave like square.

 power(base, exponent = 2) {
  let result = 1;
  for (let count = 0; count < exponent; count++) {
    result *= base;
  }
  return result;
}

console.log(power(4));
// → 16
console.log(power(2, 6));
// → 64

Summary

This chapter taught you how to write your own functions. The function keyword, when used as an expression, can create a function value. When used as a statement, it can be used to declare a binding and give it a function as its value. Arrow functions are yet another way to create functions.

 // Define f to hold a function value
const f = function(a) {
  console.log(a + 2);
};

// Declare g to be a function
function g(a, b) {
  return a * b * 3.5;
}

// A less verbose function value
let h = a => a % 3;

A key aspect in understanding functions is understanding scopes. Each block creates a new scope. Parameters and bindings declared in a given scope are local and not visible from the outside. Bindings declared with var behave differently - they end up in the nearest function scope or the global scope.

Separating the tasks your program performs into different functions is helpful. You won't have to repeat yourself as much, and functions can help organize a program by grouping code into pieces that do specific things.