Functions in JavaScript

Functions are one of the fundamental building blocks in JavaScript. They allow you to group code into reusable blocks, making your code modular, organized, and easier to maintain.

1. Defining a Function You can define a function using the function keyword followed by a name, a list of parameters enclosed in parentheses, and a block of code enclosed in curly braces.

   Syntax:

     function functionName(parameters) {
       // Code to be executed
   }
     

   Example:

     function greet(name) {
       console.log("Hello, " + name + "!");
   }
   
   greet("Alice"); // Outputs: Hello, Alice!
   greet("Bob");   // Outputs: Hello, Bob!
     

2. Function Expressions A function can also be defined as an expression. In this case, the function is assigned to a variable.

   Syntax:

     const functionName = function(parameters) {
       // Code to be executed
   };
     

   Example:

     const multiply = function(a, b) {
       return a * b;
   };
   
   console.log(multiply(3, 4)); // Outputs: 12
     

3. Arrow Functions Arrow functions provide a more concise syntax for writing function expressions. They are particularly useful for writing short functions.

   Syntax:

     const functionName = (parameters) => {
       // Code to be executed
   };
     

   Example:

     const square = x => x * x;
   
   console.log(square(5)); // Outputs: 25
     

4. Returning Values Functions can return values using the return keyword. Once a return statement is executed, the function terminates, and the returned value can be used elsewhere in your code.

   Syntax:

     function functionName(parameters) {
       return value;
   }
     

   Example:

     function add(a, b) {
       return a + b;
   }
   
   const result = add(10, 20);
   console.log(result); // Outputs: 30
     

5. Parameters and Arguments

   • Parameters are the variables listed in the function definition.
   • Arguments are the values passed to the function when it is called.

   Example:

     function greet(name, greeting = "Hello") {
       console.log(greeting + ", " + name + "!");
   }
   
   greet("Alice");               // Outputs: Hello, Alice!
   greet("Bob", "Goodbye");     // Outputs: Goodbye, Bob!
     

   In this example, greeting has a default value of “Hello” but can be overridden when calling the function.

6. Function Scope Variables declared inside a function are local to that function and cannot be accessed from outside. This is known as function scope.

   Example:

     function testScope() {
       let localVariable = "I am local";
       console.log(localVariable); // Outputs: I am local
   }
   
   testScope();
   console.log(localVariable); // Error: localVariable is not defined
     

7. IIFE (Immediately Invoked Function Expression) An IIFE is a function that runs as soon as it is defined. It is often used to create a new scope to avoid polluting the global namespace.

   Syntax:

     (function() {
       // Code to be executed immediately
   })();
     

   Example:

     (function() {
       let message = "This is an IIFE!";
       console.log(message); // Outputs: This is an IIFE!
   })();
     

8. Function Constructor Functions can also be created using the Function constructor, although this is less common and often avoided.

   Syntax:

     const func = new Function('parameters', 'code');
     

   Example:

     const subtract = new Function('a', 'b', 'return a - b;');
   
   console.log(subtract(10, 5)); // Outputs: 5