IT 231 -- Jan 25, 2024 Notes

IT 231 -- Jan 25, 2024

Review Exercises

What is the difference between a function and a method?
Answer: A function is standalone; it is not called from an object or class. A method is called from an object or a class, for example
```
parseInt("23412"); //<-- Function, which is standalone.
Math.random( )     //<-- Method, called from the Math class.
s.toUpperCase( )   //<-- Method, called from the String object s.
```
What does the toString method do for an object? Test the toString method for an Array object.
Answer: The toString method converts the object calling the toString method into a string. For example:
```
var a = [4, 2, 6, 9, 2, 4];
console.log(a.toString( ));
// Output: 4,2,6,9,2,4
```
What is an anonymous function? How do you use one?
Answer: an anonymous function is a function without a name. To use an anonymous function just place the code for the anonymous function in place of the name of the function when passing it to another function. You can also assign an anonymous function to a variable to use later.
What is arrow notation for functions?
Answer: you use a fat arrow ( => ) instead of the function keyword to define the function. See the following two sections.

Two Rules for Writing Functions in Arrow Notation

If the function has exactly one parameter, parentheses are not needed around the parameter. Otherwise parentheses are needed for the parameters. (Empty parentheses for zero parameters.)
If the body of the function consists of a one-line return statement, curly braces are not needed for the body. Otherwise curly braces are needed (for no return statement or more than one line).

Review Exercises for Functions in Arrow Notation

Rewrite these functions in arrow notation. Also, write code to test each function after it is written in arrow notation:

// a.
function inToCm(inches) {
    return inches * 2.54;
}
// Answer:
var inToCm = inches => inches * 2.54;
console.log(inToCm(2.5);
// Output: 6.35

// b.
function printGreeting( ) {
    console.log("Hello!");
}
// Answer:
var printGreeting = ( ) => { console.log("Hello!");
console.log(printGreeting( ));
// Output: Hello!

// c.
function makeGreeting(firstName) {
    return "Hello, " + firstName + ".");
}
// Answer:
var makeGreeting = firstName => "Hello, " + firstName + ".";
console.log(makeGreeting("Alice"));
// Output:
Hello, Alice. 

// d.
function ftInToIn(feet, inches) {
    return feet * 12 + inches;
}
// Answer:
var ftInToIn = (feet, inches) => feet * 12 + inches;
console.log(5, 3);
// Output: 63

SetInterval Example

This JavaScript script uses the setInterval function that calls a function repeatedly every 1,000 milliseconds (= 1 second):

function showTime( ) {
    var d = new Date( );
    console.log(d);
}
setInterval(showTime, 1000);

Rewrite this script using anonymous function instead of the showTime function. Answer:

// Using anonymous function
setInterval(function( ) {
    var d = new Date( );
    console.log(d);
}, 1000);

Rewrite it again using an anonymous function in arrow notation.

// Anonymous function using arrow notation:
setInterval(( ) => {
    var d = new Date( );
    console.log(d);
}, 1000);

Instead of printing the Date object, which invokes the Date toString method, use these methods to format the time and date like this:

10:10:00 1/17/2023

Date Methods: getDate getFullYear getHours getMinutes getMonth getSeconds
String Method: padStart
Answer:

setInterval(( ) => {
    var date = new Date( );
    var y = date.getFullYear( );
    var m = date.getMonth( );
    // Add one to date because dates are zero-based.
    var d = date.getDate( ) + 1;
    var h = date.getHours( ).
        toString( ).padStart(2, "0");
    var min = date.getMinutes( ).
        toString( ).padStart(2, "0");
    var sec = date.getSeconds( ).
        toString( ).padStart(2, "0");
    var output = 
        `${h}:${min}:${sec} ${m}/${d}/${y}`;
    console.log(output);
}, 1000);

Practice Quiz 3a

Take Practice Quiz 3a on D2L.

Looping over Array Items

Consider the array arr defined by
```
var arr = ["ibm", "microsoft", "google", "oracle"];
```
Output the array items one per line like this:
```
ibm
microsoft
google
netscape
```
Here are three for loops that accomplish this:
1. Traditional For Loop:
```
for(let i = 0; i < arr.length; i++) {
    console.log(arr[i]);
}
```
  A traditional for loop has three parts (1) initialization, (2) condition, (3) iteration, (4) body:
```
for (initialization; condition; iteration) {
    body;
}
```
  The initialization is performed once before executing the body of the for loop. The condition is checked at thebeginning of the loop, if the condition is true the body is executed again, if the condition is false, the control flow passes to the next statement after the end of the loop. The iteration is performed at the end of the loop, then control passes back to the beginning of the loop. Compare the preceding for loop with this while loop that does the same thing.
2. Modern For Loop:
```
for(let n of arr) {
    console.log(arr[i]);
}
```
3. Array forEach method. Hypermodern method.
```
function print(item) {
    console.log(item);
}
arr.forEach(print)
```

Tutorial 2b

Complete Tutorial 2b.

Using the HttpStatusCodes Module

HttpStatusCodes Example: We will use the HttpStatusCodes module for our Node servers.

Create a folder named test-http-status-codes and enter it with
```
> cd test-http-status-codes
```

Create a Node project:

> npm init

Press Enter to accept the default values for package-name, and version:

package-name: (test-http-status-codes)
version: (1.0.0)

For the entry point, use main.js:

entry point: (index.js) main.js

Accept the default values (null string) for test command, git repository, and keywords:

test command:
git repository:
keywords:

For author, enter your name:

author: Stan Smith

Click Enter to accept the default value for license:

license: (ISC)

Click Enter to answer yes for this final question:

Is this OK: (yes)

The JSON file package.json is created:

{
  "name": "http-status-codes",
  "version": "1.0.0",
  "description": "Test HTTP Status Codes Module",
  "main": "main.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "Stan Smith",
  "license": "ISC"
}

Add the http-status-codes module to your project:
```
> npm i http-status-codes
```

Create a main.js file using these JavaScript statements:

const HttpStatusCodes = require("http-status-codes");
console.log(HttpStatusCodes.StatusCodes.OK);
console.log(HttpStatusCodes.StatusCodes);

Run your main.js file:
```
node main.js
```