Functions are the building blocks of readable, maintainable, and reusable code. Functions are defined using the function keyword. Following is the syntax for defining a standard function.
function function_name() {
// function body
}
To force execution of the function, it must be called. This is called as function invocation. Following is the syntax to invoke a function.
function_name()
Example of a simple function definition
// define a function
function test() {
console.log("function called")
}
// call the function
test ()
The example defines a function test(). A pair of identifiers ( {} ) define the function body. It is also called as the function scope. A function must be invoked to force its execution.
The following output is displayed on successful execution of the above code.
function called
Function Return
When JavaScript reaches a return statement, the function will stop executing.
If the function was invoked from a statement, JavaScript will "return" to execute the code after the invoking statement.
Functions often compute a return value. The return value is "returned" back to the "caller":
Example:
Calculate the product of two numbers, and return the result:
var x = myFunction(4, 3);
// Function is called, return value will end up in x
function myFunction(a, b) {
return a * b;
// Function returns the product of a and b
}
Functions Used as Variable Values
Functions can be used the same way as you use variables, in all types of formulas, assignments, and calculations.
We'll discuss variables and functions extensively in the next series of articles
Local Variables
Variables declared within a JavaScript function, become LOCAL to the function. Local variables can only be accessed from within the function. 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.
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
Global variables
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).
It’s a good practice to minimize the use of global variables. Modern code has few or no globals. Most variables reside in their functions. Sometimes though, they can be useful to store project-level data.
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
}
Evaluation of default parameters
In JavaScript, a default parameter is evaluated every time the function is called without the respective parameter.
In the example above, anotherFunction() is called every time showMessage() is called without the text parameter.
Alternative default parameters
Sometimes it makes sense to set default values for parameters not in the function declaration, but at a later stage, during its execution.
To check for an omitted parameter, we can compare it with undefined:
function showMessage(text) {
if (text === undefined) {
text = 'empty message';
}
alert(text);
}
showMessage(); // empty message
…Or we could use the || operator:
// if text parameter is omitted or "" is passed, set it to 'empty'
function showMessage(text) {
text = text || 'empty';
...
}
Modern JavaScript engines support the nullish coalescing operator ??, it’s better when falsy values, such as 0, are considered regular:
// if there's no "count" parameter, show "unknown"
function showCount(count) {
alert(count ?? "unknown");
}
showCount(0); // 0
showCount(null); // unknown
showCount(); // unknown
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.
If we want the returned expression to wrap across multiple lines, we should start it at the same line as return. Or at least put the opening parentheses there as follows:
return (
some + long + expression
+ or +
whatever * f(a) + f(b)
)
And it will work just as we expect it to.
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.
One function – one action 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 it displays the access granted/denied message (should only perform the check and return the result).
These examples assume common meanings of prefixes. You and your team are free to agree on other meanings, but usually they’re not much different. In any case, 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.
Ultrashort function names 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 function 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.