> For the complete documentation index, see [llms.txt](https://ayugioh2003.gitbook.io/free-code-camp-practice/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ayugioh2003.gitbook.io/free-code-camp-practice/front-end-development-certification/basic-javascript/basic-javascript-30.md).

# Basic JavaScript 030

01-Comment your JavaScript Code

```javascript
// inline comment

/**
  * block comment
  *
  *
  */
```

02-Declare JavaScript Variables

* 宣告變數
* 可以直接從七種資料型態宣告
* 操作指向數據的指針，而不是資料表身
* 或直接使用 var

```javascript
// Example
var ourName;
var myName;
// Define myName below this line
```

03-Storing Values with the Assignment Operator

```javascript
// Setup
var a;
var b = 2;

// Only change code below this line
a = 7;
b = a;
```

04-Initializing Variables with the Assignment Operator

* 使用 `=` 給變數一個初始值

```javascript
// Example
var ourVar = 19;

// Only change code below this line
var a = 9;
```

05-Understanding Uninitialized Variables

* 只有宣告變數的話，初始值為 undefined
* 如果對 undefined 變數做數學運算，結果會是 NaN，代表 not a number

```javascript
// Initialize these three variables
var a=5;
var b=10;
var c="I am a";

// Do not change code below this line

a = a + 1;
b = b + 5;
c = c + " String!";
```

06-Understanding Case Sensitivity in Variables

* JS 的變數對大小寫敏感
* 但不要使用這個特性，會搞混
* 推薦駝峰命名法 camelCase
* 變數的第一個字母小寫，隨後單字的第一個字母大寫

```javascript
// Declarations
var studlyCapVar;
var properCamelCase;
var titleCaseOver;

// Assignments
studlyCapVar = 10;
properCamelCase = "A String";
titleCaseOver = 9000;
```

07-Add Two Numbers with JavaScript

```javascript
var sum = 10 + 10;
```

08-Subtract One Number from Another with JavaScript

```javascript
var difference = 45 - 33;
```

09-Multiply Two Numbers with JavaScript

```javascript
var product = 8 * 10;
```

10-Divide One Number by Another with JavaScript

```javascript
var quotient = 66 / 33;
```

11-Increment a Number with JavaScript

* i++

```javascript
var myVar = 87;

// Only change code below this line
myVar++;
```

12-Decrement a Number with JavaScript

* i--

```javascript
var myVar = 11;

// Only change code below this line
myVar--;
```

13-Create Decimal Numbers with JavaScript

```javascript
var ourDecimal = 5.7;

// Only change code below this line
var myDecimal = 5.7;
```

14-Multiply Two Decimals with JavaScript

```javascript
var product = 2.0 * 2.5;
```

15-Divide one Decimal by Another with JavaScript

```javascript
Divide one Decimal by Another with JavaScript
```

16-Finding a Remainder in JavaScript

> Example 5 % 2 = 1 because Math.floor(5 / 2) = 2 (Quotient) 2 \* 2 = 4 5 - 4 = 1 (Remainder)

```javascript
// Only change code below this line

var remainder = 11 % 3 ;
```

17-Compound Assignment With Augmented Addition

```javascript
var a = 3;
var b = 17;
var c = 12;

// Only modify code below this line

a += 12;
b += 9;
c += 7;
```

18-Compound Assignment With Augmented Subtraction

```javascript
var a = 11;
var b = 9;
var c = 3;

// Only modify code below this line

a -= 6;
b -= 15;
c -= 1;
```

19-Compound Assignment With Augmented Multiplication

```javascript
var a = 5;
var b = 12;
var c = 4.6;

// Only modify code below this line

a *= 5;
b *= 3 ;
c *= 10;
```

20-Compound Assignment With Augmented Division

```javascript
var a = 48;
var b = 108;
var c = 33;

// Only modify code below this line

a /= 12;
b /=  4;
c /= 11;
```

21-Convert Celsius to Fahrenheit

```javascript
function convertToF(celsius) {
  var fahrenheit;
  // Only change code below this line

  fahrenheit = celsius * (9/5) + 32;  

  // Only change code above this line
  return fahrenheit;
}

// Change the inputs below to test your code
convertToF(30);
```

22-Declare String Variables

```javascript
// Example
var firstName = "Alan";
var lastName = "Turing";

// Only change code below this line

var myFirstName = "Chiu";
var myLastName = "Chi Wei";
```

23-Escaping Literal Quotes in Strings

```javascript
var myStr; // Change this line

myStr = "I am a \"double quoted\" string inside \"double quotes\".";
```

24-Quoting Strings with Single Quotes

* 這功能好神奇，這樣不需要 back slash、反斜杠來跳脫字元了

```javascript
var myStr = '<a href="http://www.example.com" target="_blank">Link</a>';
```

25-Escape Sequences in Strings

* 超詭異的用法 ==

```
Code    Output
\'    单引号 single quote
\"    双引号 double quote
\\    反斜杠符 backslash
\n    换行符 newline
\r    回车符 carriage return
\t    制表符 tab
\b    退格符 backspace
\f    换页符 form feed
```

```javascript
var myStr; // Change this line

myStr = 'FirstLine\n\\SecondLine\\\rThirdLine';
```

26-Concatenating Strings with Plus Operator

```javascript
// Example
var ourStr = "I come first. " + "I come second.";

// Only change code below this line

var myStr = "This is the start. " + "This is the end.";
```

27-Concatenating Strings with the Plus Equals Operator

```javascript
// Example
var ourStr = "I come first. ";
ourStr += "I come second.";

// Only change code below this line

var myStr = "This is the first sentence. ";
myStr += "This is the second sentence.";
```

28-Constructing Strings with Variables

```javascript
// Example
var ourName = "Free Code Camp";
var ourStr = "Hello, our name is " + ourName + ", how are you?";

// Only change code below this line
var myName = "邱繼緯";
var myStr = "My name is" + myName + " and I am swell";
```

29-Appending Variables to Strings

```javascript
// Example
var anAdjective = "awesome!";
var ourStr = "Free Code Camp is ";
ourStr += anAdjective;

// Only change code below this line

var someAdjective = "酷喔喔";
var myStr = "Learning to code is ";
myStr += someAdjective;
```

30-Find the Length of a String

```javascript
// Example
var firstNameLength = 0;
var firstName = "Ada";

firstNameLength = firstName.length;

// Setup
var lastNameLength = 0;
var lastName = "Lovelace";

// Only change code below this line.

lastNameLength = lastName.length;
```