> 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 = 'Link';
```
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;
```
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