Functions

Quite often we need to perform a similar action in many places of the script.

For example, we need to show a nice-looking message when a visitor logs in, logs out and maybe somewhere else.

Functions are the main "building blocks" of the program. They allow the code to be called many times without repetition.

We've already seen examples of built-in functions, like alert(message), prompt(message, default) and confirm(question). But we can create functions of our own as well.

Function Declaration

To create a function we can use a function declaration.

It looks like this:

function showMessage() {
 alert( 'Hello everyone!' );
}

The function keyword goes first, then goes the name of the function, then a list of parameters between the parentheses (empty in the example above) and finally the code of the function, also named "the function body", between curly braces.

Our new function can be called by its name: showMessage().

For instance:

function showMessage() {
  alert( 'Hello everyone!' );
}

*!*
showMessage();
showMessage();
*/!*

The call showMessage() executes the code of the function. Here we will see the message two times.

This example clearly demonstrates one of the main purposes of functions: to avoid code duplication.

If we ever need to change the message or the way it is shown, it's enough to modify the code in one place: the function which outputs it.

Local variables

A variable declared inside a function is only visible inside that function.

For example:

function showMessage() {
*!*
  let message = "Hello, I'm JavaScript!"; // local variable
*/!*

  alert( message );
}

showMessage(); // Hello, I'm JavaScript!
alert( message ); // <-- Error! The variable is local to the function

Outer variables

A function can access an outer variable as well, for example:

let *!*userName*/!* = 'John';

function showMessage() {
  let message = 'Hello, ' + *!*userName*/!*;
  alert(message);
}

showMessage(); // Hello, John

The function has full access to the outer variable. It can modify it as well.

For instance:

let *!*userName*/!* = 'John';

function showMessage() {
  *!*userName*/!* = "Bob"; // (1) changed the outer variable

  let message = 'Hello, ' + *!*userName*/!*;
  alert(message);
}

alert( userName ); // *!*John*/!* before the function call
showMessage();
alert( userName ); // *!*Bob*/!*, the value was modified by the function

The outer variable is only used if there's no local one. So an occasional modification may happen if we forget let.

If a same-named variable is declared inside the function then it shadows the outer one. For instance, in the code below the function uses the local userName. The outer one is ignored:

let userName = 'John';

function showMessage() {
*!*
  let userName = "Bob"; // declare a local variable
*/!*

  let message = 'Hello, ' + userName; // *!*Bob*/!*
  alert(message);
}

// the function will create and use its own userName
showMessage();
alert( userName ); // *!*John*/!*, unchanged, the function did not access the outer variable

Variables declared outside of any function, such as the outer userName in the code above, are called *global*.

Global variables are visible  from any function (unless shadowed by locals).

Usually, a function declares all variables specific to its task. Global variables only store project-level data, so when it's important that these variables are accesible from anywhere. Modern code has few or no globals. Most variables reside in their functions.

Parameters

We can pass arbitrary data to functions using parameters (also called function arguments) .

In the example below, the function has two parameters: from and text.

function showMessage(*!*from, text*/!*) { // arguments: from, text
  alert(from + ': ' + text);
}

*!*
showMessage('Ann', 'Hello!'); // Ann: Hello! (*)
showMessage('Ann', "What's up?"); // Ann: What's up? (**)
*/!*

When the function is called in lines (*) and (**), the given values are copied to local variables from and text. Then the function uses them.

Here's one more example: we have a variable from and pass it to the function. Please note: the function changes from, but the change is not seen outside, because a function always gets a copy of the value:

function showMessage(from, text) {

*!*
  from = '*' + from + '*'; // make "from" look nicer
*/!*

  alert( from + ': ' + text );
}

let from = "Ann";
showMessage(from, "Hello"); // *Ann*: Hello
// the value of "from" is the same, the function modified a local copy
alert( from ); // Ann

Default values

If a parameter is not provided, then its value becomes undefined.

For instance, the aforementioned function showMessage(from, text) can be called with a single argument:

showMessage("Ann");

That's not an error. Such a call would output "Ann: undefined". There's no text, so it's assumed that text === undefined.

If we want to use a "default" text in this case, then we can specify it after =:

function showMessage(from, *!*text = "no text given"*/!*) {
  alert( from + ": " + text );
}

showMessage("Ann"); // Ann: no text given

Now if the text parameter is not passed, it will get the value "no text given"

Here "no text given" is a string, but it can be a more complex expression, which is only evaluated and assigned if the parameter is missing. So, this is also possible:

function showMessage(from, text = anotherFunction()) {
  // anotherFunction() only executed if no text given
  // its result becomes the value of text
}

 In JavaScript, a default parameter is evaluated every time the function is called without the respective parameter. In the example above, anotherFunctions() is called everytime someMessage() is called without the text parameter. This is in contrast to some other languages like Python, where any default parameters are evaluated only once during the initial interpretation.

Old editions of JavaScript did not support default parameters. So there are alternative ways to support them, that you can find mostly in the old scripts.

For instance, an explicit check for being undefined:

function showMessage(from, text) {
*!*
  if (text === undefined) {
    text = 'no text given';
  }
*/!*

  alert( from + ": " + text );
}

...Or the || operator:

function showMessage(from, text) {
  // if text is falsy then text gets the "default" value
  text = text || 'no text given';
  ...
}

Returning a value

A function can return a value back into the calling code as the result.

The simplest example would be a function that sums two values:

function sum(a, b) {
  *!*return*/!* a + b;
}

let result = sum(1, 2);
alert( result ); // 3

The directive return can be in any place of the function. When the execution reaches it, the function stops, and the value is returned to the calling code (assigned to result above).

There may be many occurrences of return in a single function. For instance:

function checkAge(age) {
  if (age > 18) {
*!*
    return true;
*/!*
  } else {
*!*
    return confirm('Do you have permission from your parents?');
*/!*
  }
}

let age = prompt('How old are you?', 18);

if ( checkAge(age) ) {
  alert( 'Access granted' );
} else {
  alert( 'Access denied' );
}

It is possible to use return without a value. That causes the function to exit immediately.

For example:

function showMovie(age) {
  if ( !checkAge(age) ) {
*!*
    return;
*/!*
  }

  alert( "Showing you the movie" ); // (*)
  // ...
}

In the code above, if checkAge(age) returns false, then showMovie won't proceed to the alert.

A function with an empty return or without it returns undefined

If a function does not return a value, it is the same as if it returns undefined:

function doNothing() { /* empty */ }
alert( doNothing() === undefined ); // true

An empty return is also the same as return undefined:

function doNothing() {
  return;
}

alert( doNothing() === undefined ); // true

Never add a newline between return and the value

For a long expression in return, it might be tempting to put it on a separate line, like this:

return
(some + long + expression + or + whatever * f(a) + f(b))

That doesn't work, because JavaScript assumes a semicolon after return. That'll work the same as:

return*!*;*/!*
(some + long + expression + or + whatever * f(a) + f(b))

So, it effectively becomes an empty return. We should put the value on the same line instead.

Naming a function

Functions are actions. So their name is usually a verb. It should be brief, as accurate as possible and describe what the function does, so that someone reading the code gets an indication of what the function does.

It is a widespread practice to start a function with a verbal prefix which vaguely describes the action. There must be an agreement within the team on the meaning of the prefixes.

For instance, functions that start with "show" usually show something.

Function starting with...

• "get…" - return a value,
• "calc…" - calculate something,
• "create…" - create something,
• "check…" - check something and return a boolean, etc.

Examples of such names:

showMessage(..)     // shows a message
getAge(..)          // returns the age (gets it somehow)
calcSum(..)         // calculates a sum and returns the result
createForm(..)      // creates a form (and usually returns it)
checkPermission(..) // checks a permission, returns true/false

With prefixes in place, a glance at a function name gives an understanding what kind of work it does and what kind of value it returns.

A function should do exactly what is suggested by its name, no more. 

Two independent actions usually deserve two functions, even if they are usually called together (in that case we can make a 3rd function that calls those two).

A few examples of breaking this rule:

• getAge - would be bad if it shows an alert with the age (should only get).
• createForm - would be bad if it modifies the document, adding a form to it (should only create it and return).
• checkPermission - would be bad if displays the access granted/denied message (should only perform the check and return the result).

These examples assume common meanings of prefixes. What they mean for you is determined by you and your team. Maybe it's pretty normal for your code to behave differently. But you should have a firm understanding of what a prefix means, what a prefixed function can and cannot do. All same-prefixed functions should obey the rules. And the team should share the knowledge.

Functions that are used very often sometimes have ultrashort names.

For example, the jQuery framework defines a function with $. The LoDash library has its core function named _.

These are exceptions. Generally functions names should be concise and descriptive.

Functions == Comments

Functions should be short and do exactly one thing. If that thing is big, maybe it's worth it to split the function into a few smaller functions. Sometimes following this rule may not be that easy, but it's definitely a good thing.

A separate function is not only easier to test and debug -- its very existence is a great comment!

For instance, compare the two functions showPrimes(n) below. Each one outputs prime numbers up to n.

The first variant uses a label:

function showPrimes(n) {
  nextPrime: for (let i = 2; i < n; i++) {

    for (let j = 2; j < i; j++) {
      if (i % j == 0) continue nextPrime;
    }

    alert( i ); // a prime
  }
}

The second variant uses an additional function isPrime(n) to test for primality:

function showPrimes(n) {

  for (let i = 2; i < n; i++) {
    *!*if (!isPrime(i)) continue;*/!*

    alert(i);  // a prime
  }
}

function isPrime(n) {
  for (let i = 2; i < n; i++) {
    if ( n % i == 0) return false;
  }
  return true;
}

The second variant is easier to understand, isn't it? Instead of the code piece we see a name of the action (isPrime). Sometimes people refer to such code as self-describing.

So, functions can be created even if we don't intend to reuse them. They structure the code and make it readable.

Summary

A function declaration looks like this:

function name(parameters, delimited, by, comma) {
  /* code */
}

• Values passed to a function as parameters are copied to its local variables.
• A function may access outer variables. But it works only from inside out. The code outside of the function doesn't see its local variables.
• A function can return a value. If it doesn't, then its result is undefined.

To make the code clean and easy to understand, it's recommended to use mainly local variables and parameters in the function, not outer variables.

It is always easier to understand a function which gets parameters, works with them and returns a result than a function which gets no parameters, but modifies outer variables as a side-effect.

Function naming:

• A name should clearly describe what the function does. When we see a function call in the code, a good name instantly gives us an understanding what it does and returns.
• A function is an action, so function names are usually verbal.
• There exist many well-known function prefixes like create…, show…, get…, check… and so on. Use them to hint what a function does.

Functions are the main building blocks of scripts. Now we've covered the basics, so we actually can start creating and using them. But that's only the beginning of the path. We are going to return to them many times, going more deeply into their advanced features.