JavaScript Function Expressions

There are two different ways of creating functions in JavaScript. We can use function declaration and function expression. The difference between them is the function name, which can be omitted in function expressions to create anonymous functions.

Here is the syntax for Function Declaration:

Javascript function declaration

function welcome() { console.log("Welcome to W3Docs!"); } welcome();

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And this is the syntax for Function Expression:

javascript function expression

let welcome = function () { console.log("Welcome to W3Docs!"); }; welcome();

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The function is assigned to the variable, like any other value no matter how the function is defined because it is just a value stored in the variable welcome.

We can also show that value using console.log:

Javascript function show the value in console

function welcome() { console.log("Welcome to W3Docs"); } console.log(welcome); // it shows code of the function welcome();

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JavaScript is not like the programming languages where any mention of a function name causes its execution. In JavaScript, a function is a value and we can deal with it as a value. The code in the example above shows its string representation, which is the source code.

A function is a special value, we can call it like welcome(). It is still a value and we can work with it the way we work with other values.

We can even copy a function to another variable like this:

Javascript function declaration

function welcome() { // (1) create the function console.log("Welcome to W3Docs"); } let anotherFunc = welcome; // (2) copy anotherFunc(); // Welcome to W3Docs //it works welcome(); // Welcome to W3Docs // this works too

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Let’s discuss what happens above in detail:

1. The Function Declaration (1) creates the function putting it into the variable named welcome.
2. Line (2) copies the variable welcome into the variable anotherFunc.
3. It is possible to call the function as both welcome() and anotherFunc().

Javascript function expression

let welcome = function () { console.log("Welcome to W3Docs"); }; let anotherFunc = welcome; anotherFunc();

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A semicolon at the end

You can ask, why Function Expression has a semicolon at the end, but Function Declaration does not:

function welcome() {
  // ...
}
let welcome = function () {
  // ...
};

The reason is there’s no need for semicolons at the end of code blocks and syntax structures that use them as if { ... }, for { }, function f { }, and so on.

A Function Expression can be used inside the statement: let welcome = ...;, as a value, but not a code block. It is recommended to put the semicolon at the end of statements, no matter what the value is. That is why the semicolon here is not related to the Function Expression itself.

Callback functions

A function passed to another function is called a “callback” in JavaScript.

There are examples of passing functions as values and using function expressions presented below.

The function should ask the siteQuestion and call answerYes() or answerNo() depending on the user’s answer:

Javascript callback function

function callbackFunc(siteQuestion, answerYes, answerNo) { if (confirm(siteQuestion)) { answerYes() } else { answerNo(); } } function chooseOk() { console.log("It’s great."); } function chooseCancel() { console.log("Try again"); } // use answerOk, answerCancel functions passed as arguments to the request callbackFunc("Are you familiar with JS function decalarations?", chooseOk, chooseCancel);

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In practice, these kinds of functions are quite useful. The main difference between a real-life callbackFunc and the example above is that real-life functions use more difficult ways to interact with the user than a simple confirm.

We pass a function expecting that it will be “called back” later if necessary. In this case, chooseOk becomes the callback for “yes” answer, and chooseCancel for “no”.

We can use Function Expressions if we want to write the same function much shorter:

Javascript callback function

function callbackFunc(siteQuestion, answerYes, answerNo) { if (confirm(siteQuestion)) { answerYes(); } else { answerNo(); } } callbackFunc( "Are you familiar with JS function declarations?", function () { console.log("It’s great."); }, function () { console.log("Try again"); } );

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Functions are declared inside the callbackFunc(...) call and they have no name, which is why they are called anonymous. These kinds of functions are not accessible outside of callbackFunc. This kind of code appears in our scripts very naturally.

Structured values like strings or numbers represent the data. A function can be considered an action. We can proceed between variables and run when we want.

Function Expression and Function Declaration

The main difference between Function Declarations and Expressions is the syntax.

Function Expression defines a function, created inside an expression or another syntax construct. The presented function is created at the right side of the “assignment expression” =

// Function Expression
let sum = function (a, b) {
  return a + b;
};

A Function Expression works almost the same way as a function declaration or a function statement, the only difference is that a function name is not started in a function expression. The function expressions run as soon as they are defined.

Function Declaration defines a named function, declared as a separate statement, in the main code flow. Just as Variable Declaration must start with “var”, Function Declaration must start with “function”.

// Function Declaration
function sum(a, b) {
  return a + b;
}

The more delicate difference between the functions is when a function is created by the JavaScript engine.

Once the execution flow passes to the right side of the assignment let sum = function..., the function is created and can be used.

Function Declarations can be called earlier than they are defined. For instance, no matter where a global Function Declaration is, it is visible in the whole script. The reason is the internal algorithm.

When all Function Declarations are processed, the code is executed and has access to these functions.

For instance, this works:

Javascript function declaration

welcome("W3Docs"); // Welcome to W3Docs function welcome(siteName) { console.log(`Welcome to, ${siteName}`); }

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But it would not work If it was a Function Expression:

Javascript function expression

welcome("W3Docs"); // error! let welcome = function (siteName) { // (*) there is no more magic console.log(`Welcome to ${siteName}`); };

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Function Expressions will be created when the implementation reaches them, which would happen only in the line (*).

One more special attribute of Declaration is its block scope.

For example, let’s suppose that we need to declare a function welcome() depending on the age variable that we get during runtime, then we plan to use it later.

If we use Function Declaration in strict mode, it won’t work as planned:

Javascript function declaration in strict mode

"use strict" let number = prompt("Guess the number", ""); // conditionally declare a function if (number == 10) { function guessNumber() { // A function declaration is available everywhere console.log("it’s right!"); //in the block where it is declared. } } else { function guessNumber() { console.log("it’s wrong!"); } } //we cannot see function declarations inside them, //because here we have left the braces guessNumber(); // Error: guessNumber is not defined

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What can we do for making welcome visible outside of if?

The best proposal would be to use a Function Expression and assign welcome to the variable that is declared outside of if.

This code works as planned:

Javascript function expression in strict mode

"use strict" let number = prompt("Guess the number", ""); let guessNumber; // conditionally declare a function if (number == 10) { guessNumber = function () { console.log("it’s right!"); } } else { guessNumber = function () { console.log("it’s wrong!"); } } guessNumber(); // now it’s work

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But we could also simplify it using a question mark operator ?:

Javascript function expression

let number = prompt("Guess the number", ""); let guessNumber = (number == 10) ? function () { console.log("it’s right!"); } : function () { console.log("it’s wrong!"); }; guessNumber(); // it’s work

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When to choose Function Declaration against Function Expression?

When we need to declare a function, Function Declaration syntax is the first to consider. With it, we are free to organize our code the way we want because we can call such functions before they are declared.

One more advantage is readability, as it’s easier to look up function f(…) {…} in the code than let f = function(…) {…};. Function Declarations are more impressive. But we need to use Function Declaration in case we need a conditional declaration.