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JavaScript

Functions - Reusable Blocks of Code

Easy
10 min

What Is a Function?

A function is a block of instructions in your code that you can use over and over again. You give the function a name, and when you call that name, the code inside the function will execute. Functions can take inputs (we call these parameters) and can give back an output (called a return value).

Declaring a Function

To create a function, you can use the function keyword. For example, this function prints a greeting:

function sayHello() {
  console.log("Hello, Function!");
}

Here, we have:

  • Declaration: The keyword function tells JavaScript we are creating a function.
  • Name: The function is named sayHello.
  • Body: The code inside the curly braces {} is what the function does. In this case, it prints a message.

Calling a Function

Once a function is declared, you can run it (or “call” it) by writing its name followed by parentheses:

sayHello();

This tells JavaScript, “Please run the instructions inside the function called sayHello.”

Functions with Parameters and Return Values

Functions can also accept inputs. For example, here’s a function that adds two numbers:

function addNumbers(a, b) {
  return a + b;
}

In this case:

  • Parameters: The function takes two inputs, a and b.
  • Return Value: The function returns the result of adding a and b.

You can call this function and store its result in a variable:

let sum = addNumbers(3, 4);
console.log(sum); // prints 7

Exercises

Simple Function

  • Create a function called sayHello that prints "Hello, Function!" to the console.
  • After declaring the function, call it so that the message is printed.

Function with Parameters

  • Create a function named addNumbers that takes two parameters.
  • The function should return the sum of the two parameters
  • .Call the function with the numbers 3 and 5, store the result in a variable called result, and print result using console.log.

Calculating the Area of a Rectangle

  • Create a function named calculateArea using the function keyword.
  • It should take two parameters: width and height.
  • The function should return the product of width and height.
  • After declaring the function, call it with the numbers 7 and 3, and print the result using console.log
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