Write Reusable JavaScript with Functions:

In JavaScript, we can divide up our code into reusable parts called functions.

Here's an example of a function:

function functionName() {
  console.log("Hello World");
}

You can call or invoke this function by using its name followed by parentheses, like this: functionName(); Each time the function is called it will print out the message Hello World on the dev console. All of the code between the curly braces will be executed every time the function is called.

Passing Values to Functions with Arguments:

Parameters are variables that act as placeholders for the values that are to be input to a function when it is called. When a function is defined, it is typically defined along with one or more parameters. The actual values that are input (or "passed") into a function when it is called are known as arguments.

Here is a function with two parameters, param1 and param2:

function testFun(param1, param2) {
  console.log(param1, param2);
}

Then we can call testFun like this: testFun("Hello", "World");. We have passed two string arguments, Hello and World. Inside the function, param1 will equal the string Hello and param2 will equal the string World. Note that you could call testFun again with different arguments and the parameters would take on the value of the new arguments.

Return a Value from a Function with Return:

We can pass values into a function with arguments. You can use a return statement to send a value back out of a function.

Example

function plusThree(num) {
  return num + 3;
}

const answer = plusThree(5);

answer has the value 8. 

plusThree takes an argument for num and returns a value equal to num + 3.