Javascript Functions

Question 1

A JavaScript function is a block of code designed to perform a particular task.

A JavaScript function is executed when “something” invokes it (calls it).

This example calls a function which performs a calculation, and returns the result:
Funciton with parameters of p1 and p2, function returns product of p1 and p2.
user enter 4 for p1 and 3 for p2.

Answer 1

script>
function myFunction(p1, p2) {
  return p1 * p2;
}
document.getElementById("demo").innerHTML = myFunction(4, 3);

Question 2

JavaScript Function Syntax
A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses ().

Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).

The parentheses may include parameter names separated by commas:
(parameter1, parameter2, …)

The code to be executed, by the function, is placed inside curly brackets: {}

Function parameters are listed inside the parentheses () in the function definition.

Function arguments are the values received by the function when it is invoked.

Inside the function, the arguments (the parameters) behave as local variables.

A Function is much the same as a Procedure or a Subroutine, in other programming languages.

Answer 2

function name(parameter1, parameter2, parameter3) {
  // code to be executed
}

Question 3

Function Invocation

The code inside the function will execute when “something” invokes (calls) the function:

Answer 3

• When an event occurs (when a user clicks a button)
• When it is invoked (called) from JavaScript code
• Automatically (self invoked)

Question 4

Function Return
When JavaScript reaches a return statement, the function will stop executing.

If the function was invoked from a statement, JavaScript will “return” to execute the code after the invoking statement.

Functions often compute a return value. The return value is “returned” back to the “caller”:

The var x has the value of myFuction(4,3).
The Function has the name myFunction with () of (a,b) and a return of a * b.

Answer 4

script>
var x = myFunction(4, 3);
document.getElementById("demo").innerHTML = x;

function myFunction(a, b) {
  return a * b;
}

Question 5

Why Functions?
You can reuse code: Define the code once, and use it many times.

You can use the same code many times with different arguments, to produce different results.

This example calls a function to convert from Fahrenheit to Celsius:

The Function has a name of toCelsius(f) and a return of (5/9 * (f-32).
The user enters 77 as the Fahrenheit.

Answer 5

script>
function toCelsius(f) {
  return (5/9) * (f-32);
}
document.getElementById("demo").innerHTML = toCelsius(77);

Question 6

The () Operator Invokes the Function
Using the example above, toCelsius refers to the function object, and toCelsius() refers to the function result.

Accessing a function without () will return the function definition instead of the function result:

Function name: to Celsius(f).
Function return: (5/9) * (f-32)

Browser displays this: function toCelsius(f) { return (5/9) * (f-32); }

What’s missing?

Answer 6

script>
function toCelsius(f) {
  return (5/9) * (f-32);
}
document.getElementById("demo").innerHTML = toCelsius;

Question 7

Functions Used as Variable Values
Functions can be used the same way as you use variables, in all types of formulas, assignments, and calculations.

You can use the function directly, as a variable value:

Instead of using a variable to store the return value of a function:

Answer 7

var x = toCelsius(77);
var text = "The temperature is " + x + " Celsius";

var text = “The temperature is “ + toCelsius(77) + “ Celsius”;

document.getElementById(“demo”).innerHTML =
“The temperature is “ + toCelsius(77) + “ Celsius”;

function toCelsius(fahrenheit) {
  return (5/9) * (fahrenheit-32);

Question 8

Local Variables
Variables declared within a JavaScript function, become LOCAL to the function.

Local variables can only be accessed from within the function.
Ex:
// code here can NOT use carName

function myFunction() {
  var carName = "Volvo";
  // code here CAN use carName
}

// code here can NOT use carName

BROWSER DISPLAYS THIS: Outside myFunction() carName is undefined.

string Volvo

undefined

Answer 8

script>
myFunction();

function myFunction() {
  var carName = "Volvo";
  document.getElementById("demo1").innerHTML =
  typeof carName + " " + carName;
}

document.getElementById(“demo2”).innerHTML =
typeof carName;

Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Local variables are created when a function starts, and deleted when the function is completed.