Functions in Javascript (Declaration, Expression, Arrow)

There are three ways of using functions and each has its own name, one is called “Function Declarations” and another is “Function Expressions”. The last one to be discussed is “Arrow Function”, which has the shortest syntax for a function.

The Syntactical differences between them are like so :

Function Declaration declared as a separate statement, in the main code flow.

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

Function Expression is created inside an expression or inside another syntax construct. The function below is treated like a value which gets assigned into a variable – together when called with ( ), will become an executable function.

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

Arrow function offers a shorter syntax like so :

let sum = (a, b) => a + b; // one line of expression does not require "return" keyword, because it is implicit

If we have only one argument, then parentheses can be omitted, making that even shorter:

let displayA = a => alert("this is ", a);

If there are no arguments, parentheses should be empty (but they should be present):

let sayHi = () => alert("Hello!");

If there is more than one line inside the code block encapsulated by the pair of { } , we would need to use the “return” keyword, like so :

let sum = (a, b) => {
   let result = a + b;
   return result;

Now, let’s discuss about the the more subtle difference between the Function Declaration and Function Expression. It has to do with the execution time of the function.

A Function Expression is created when the execution reaches it and is usable from then on.

Once the execution flow passes to the right side of the assignment,

let sum = function(a,b) { ...

at that point, the function is created and can be used thereafter (assigned, called etc).

Function Declarations are different.

A Function Declaration is usable in the whole script/code block.

In other words, when JavaScript prepares to run the script or a code block, it first looks for Function Declarations in it and creates the functions. Much like var that is hoisted; declaring a variable anywhere in the code is equivalent to declaring it at the top.

And after all of the Function Declarations are processed, the execution moves on.

As a result, a function declared as a Function Declaration can be called earlier than it is defined.

For example, this works:

sayHi("Pillow"); // Hello, Pillow 

function sayHi(name) { 
   alert( `Hello, ${name}` ); 

The Function Declaration sayHi is created when JavaScript is preparing to start the script and is visible everywhere in it.

If it were a Function Expression, then it wouldn’t work:

sayHi("Pillow"); // Error! 

// so far, the execution has never encountered the sayHi function

let sayHi = function(name) {  // (*) execution happens here and only in lines below, the sayHi can be used

   alert( `Hello, ${name}` );

Function Expressions are created when the execution reaches them. That would happen only in the line marked with (*) .

Another post will cover specifically about Arrow function, but the gists about Arrow function are :

  • Does not have its own this (inherit it from its nearest object-bound parent, read more here)
  • Does not have arguments
  • Can’t be called with new
  • Does not have super


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