Functions are created/ defined then they are called.

Defining a function:

// Function definition

function calculateBill() {
	// this is the function body
	console.log('running calculateBill');
}

Calling a function:

// Function call or run

calculateBill(); // running calculateBill (returns undefined)

Variables created inside a function are not available outside the function. e.g. total above.

It is a temporary variable. After running of the function is complete, the variable is cleaned up or garbage-collected.

Returning value from function:

function calculateBill() {
	const total = 100 * 1.13;
	return total; // total is returned
}

calculateBill(); // returns 112.999999999

Capturing returned value from a function into a variable:

const myTotal = calculateBill(); (myTotal will have value 112.999999999)