## Javascript Course

Simple arithmetic operations with numbers can be performed with specific operators: Subtraction (-), addition (+), multiplication (*), division (/) and modulo (%) the rest of the division. See the lesson from: coursesweb.net/javascript/operators

- Example, gets the sum of three numbers, then the rest of division with 3.
```const n1 =2;
const n2 =9;

//sum
let sum_n = n1 +n2 +15; // 26

//rest of division with 3
let rest3 = sum_n %3; // 2

document.write('Sum (n1+n2+15) = '+sum_n+'<br>Rest of this sum divided by 3 is: '+rest3);
```

### Numeric strings

Numeric strings (numbers added between quotes) cannot be used to perform correctly mathematical operations.
JavaScript uses the + operator for both addition and concatenation.
Numbers are added. Strings are concatenated

- Example, trying to add a numeric string to a number, it results a string with the numbers joined:
```let n1 ='21';
let n2 =9;

//sum
let sum_n = n1 +n2; //219

document.write("Sum ('21'+ 9) ="+sum_n);
```

Before performing mathematical operations with numeric strings, they must be converted to `Number`.
The simplest way to convert a numeric string to Number is by: multiplying the string by 1, or with the `Number()` function.

- Example:
```let n1 ='21';
let n2 =9;

n1 = n1*1; //converts to number
let sum_n = n1 +n2; //30

document.write('Sum (21+9) = '+sum_n);
```
The same example, with the Number() function:
```let n1 ='21';
let n2 =9;

n1 = Number(n1); //converts to number
let sum_n = n1 +n2; //30

document.write('Sum of (21+9) = '+sum_n);
```

### Functions for the Number object

Numbers without decimal are considered Integer, and the decimal numbers are considered Float.
The negative numbers are added with the minus sign (-).
Number object has some methods for numbers.

`Number.isInteger(nr)` - returns True if 'nr' is an Integer, otherwise, False.
```var n1 = 90;
var n2 = -25;
var n3 = 90.23;
var n4 ='23';

console.log(Number.isInteger(n1)); // true
console.log(Number.isInteger(n2)); // true
console.log(Number.isInteger(n3)); // false
console.log(Number.isInteger(n4)); // false
```
`Number(sn)` - converts a numeric string 'sn' to number. returns NaN (Not a Number) if the string is not numeric.
```var n1 ='57.98';
var n2 ='23 str';

console.log(Number(n1)); // 57.98
console.log(Number(n2)); // NaN
```
`parseFloat(sn)` - parses an argument and returns a floating point number.
```var nr ='13.56';
nr = parseFloat(nr); //number 13.56
var sum_n = nr +8.2;

document.write('- Suma: nr +8 = '+sum_n); //21.759999999999998
```
`parseInt(sn)` - parses a string argument and returns a string with an integer number.
```var n1 ='25.89';
var n2 ='-34.8 23';

console.log(parseInt(n1)); // 25
console.log(parseInt(n2)); // -34
```
`nr.toFixed(d)` - returns a string representing the given number using fixed-point notation, with the specified number of decimals 'd'.
- This method is applied to numbers, not to 'numeric strings'.
```var n1 = 92;
var n2 = -25.5689;
var n3 = 90.2378;

console.log(n1.toFixed(2)); // 92.00
console.log(n2.toFixed(1)); // -25.6
console.log(n3.toFixed(2)); // 90.24
```
`nr.toPrecision(n)` - returns a string with the number from 'nr', having:
- for 'nr'>1, the specified length of digits 'n'.:
- for 'nr'<1, the specified length of decimals 'n'.
```var n1 = 24;
var n2 = -3.23;
var n3 = 0.46;

console.log(n1.toPrecision(3)); // 24.0
console.log(n2.toFixed(1)); // -3.2
console.log(n3.toFixed(3)); // 0.460
```
`nr.toString()` - returns a string with the number from 'nr'.
```var n1 = 12.5;
var sum_n = n1.toString() +3;
var ex2 = (12 + 9).toString();

console.log(sum_n); // 12.53
console.log(ex2); // 21
```

#### The Number() function with the Date object

The `Number()` applied with the Date object returns the number of milliseconds since 1.1.1970 till the time from the Date object.
```var dt1 = new Date('2018-07-15');
var dt2 = new Date('2018-07-15 11:13:00');

console.log(Number(dt1)); // 1531612800000
console.log(Number(dt2)); // 1531642380000
```

#### Daily Test with Code Example

##### PHP-MySQL
Which tag defines the clickable areas inside the image map?
<map> <img> <area>
```<img src="image.jpg" usemap="#map1">
<map name="map1">
<area shape="rect" coords="9, 120, 56, 149" href="#">
<area shape="rect" coords="100, 200, 156, 249" href="#">
</map>```
Which CSS property defines what is done if the content in a box is too big for its defined space?
display overflow position
```#id {
overflow: auto;
}```
Click on the event which is triggered when the mouse is positioned over an object.
onclick onmouseover onmouseout
```document.getElementById("id").onmouseover = function(){
document.write("Have Good Life");
}```
Indicate the PHP variable that contains data added in URL address after the "?" character.
\$_SESSION \$_GET \$_POST
```if(isset(\$_GET["id"])) {
echo \$_GET["id"];
}```
Numbers in JavaScript

#### Last accessed pages

1. Counter Page Visits (1134)
2. Routing (384)
3. Courses Web: PHP-MySQL JavaScript Node.js Ajax HTML CSS (140040)
4. Uploading multiple files (1395)
5. Moving html element to a random direction (5168)

#### Popular pages this month

1. Courses Web: PHP-MySQL JavaScript Node.js Ajax HTML CSS (497)
2. PHP-MySQL free course, online tutorials PHP MySQL code (91)
3. Read Excel file data in PHP - PhpExcelReader (55)
4. The Mastery of Love (43)
5. The Fifth Agreement (42)