FilesExpand file tree

 master

/

variable.js

Copy path

Blame

More file actions

Blame

More file actions

Latest commit

 

History

History

History

113 lines (98 loc) · 3.13 KB

 master

/

variable.js

Copy path

Top

File metadata and controls

• Code
• Blame

113 lines (98 loc) · 3.13 KB

Raw

Copy raw file

Download raw file

Open symbols panel

Edit and raw actions

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107

108

109

110

111

112

113

// 1. Use strict

// added in ES 5

// use this for Vanila Javascript.

'use strict';

// 2. Variable, rw(read/write)

// let (added in ES6)

let globalName = 'global name';

{

let name = 'ellie';

console.log(name);

name = 'hello';

console.log(name);

console.log(globalName);

}

console.log(name);

console.log(globalName);

// var (don't ever use this!)

// var hoisting (move declaration from bottom to top)

// has no block scope

{

age = 4;

var age;

}

console.log(age);

// 3. Constant, r(read only)

// use const whenever possible.

// only use let if variable needs to change.

const daysInWeek = 7;

const maxNumber = 5;

// Note!

// Immutable data types: premitive types, frozen objects (i.e. object.freeze())

// Mutable data types: all objects by default are mutable in JS

// favor immutable data type always for a few reasons:

// - security

// - thread safety

// - reduce human mistakes

// 4. Variable types

// primitive, single item: number, string, boolean, null, undefined, symbol

// object, box container

// function, first-class function

// number

const count = 17; // integer

const size = 17.1; // decimal number

console.log(`value: ${count}, type: ${typeof count}`);

console.log(`value: ${size}, type: ${typeof size}`);

// number - speicla numeric values: infinity, -infinity, NaN

const infinity = 1 / 0;

const negativeInfinity = -1 / 0;

const nAn = 'not a number' / 2;

console.log(infinity);

console.log(negativeInfinity);

console.log(nAn);

// bigInt (fairly new, don't use it yet)

const bigInt = 1234567890123456789012345678901234567890n; // over (-2**53) ~ 2*53)

console.log(`value: ${bigInt}, type: ${typeof bigInt}`);

// string

const char = 'c';

const brendan = 'brendan';

const greeting = 'hello ' + brendan;

console.log(`value: ${greeting}, type: ${typeof greeting}`);

const helloBob = `hi ${brendan}!`; //template literals (string)

console.log(`value: ${helloBob}, type: ${typeof helloBob}`);

console.log('value: ' + helloBob + ' type: ' + typeof helloBob);

// boolean

// false: 0, null, undefined, NaN, ''

// true: any other value

const canRead = true;

const test = 3 < 1; // false

console.log(`value: ${canRead}, type: ${typeof canRead}`);

console.log(`value: ${test}, type: ${typeof test}`);

// null

let nothing = null;

console.log(`value: ${nothing}, type: ${typeof nothing}`);

// undefined

let x;

console.log(`value: ${x}, type: ${typeof x}`);

// symbol, create unique identifiers for objects

const symbol1 = Symbol('id');

const symbol2 = Symbol('id');

console.log(symbol1 === symbol2);

const gSymbol1 = Symbol.for('id');

const gSymbol2 = Symbol.for('id');

console.log(gSymbol1 === gSymbol2); // true

console.log(`value: ${symbol1.description}, type: ${typeof symbol1}`);

// object, real-life object, data structure

const ellie = { name: 'ellie', age: 20 };

ellie.age = 21;

// 5. Dynamic typing: dynamically typed language

let text = 'hello';

console.log(text.charAt(0)); //h

console.log(`value: ${text}, type: ${typeof text}`);

text = 1;

console.log(`value: ${text}, type: ${typeof text}`);

text = '7' + 5;

console.log(`value: ${text}, type: ${typeof text}`);

text = '8' / '2';

console.log(`value: ${text}, type: ${typeof text}`);

console.log(text.charAt(0));