Why does data need a type? The new guy never gets it

Numbers and Strings

Why do we have a 1 and a ‘1’?

Function of different

• Numbers are numbers, strings are strings. Be careful

• Numbers can add, subtract, multiply and divide, but strings cannot

• Strings can represent phone numbers, not numbers

Different types of storage

• In JS, numbers are stored as 64-bit floating point numbers
• In JS, strings are stored in a form like UTF8 (UCS-2)

How to Store numbers

Convert decimal to binary

binary

10 to 2

• 31 becomes binary

• After some trial

• So 31(decimal) = 01 1111(binary)
• Isn’t it a formula? Programmers never use formulas

2 to 10

• The value of 100011 is decimal
• Multiply each of them by 2 to the N, and then add them up
• 100011 = 2^5 + 2^1 + 2^0 = 35

Binary is represented in hexadecimal

Why hexadecimal

• Because binary is too slow to write: 011110001011010
• Remember 8, 4, 2, 1 for X, X, X, X
• Every four bits from right to left are changed to 011110001011010
• Get 3,12,5,10; Change the number greater than 9 to ABCDEF
• So I get 3C5A

• You can also use calculator programmer mode
• HEX indicates hexadecimal and BIN indicates binary
• OCT means base 8, DEC means base 10

How to store characters

Why don’t you just turn it into a number? But there are thousands of characters

How do I store a, B, C, 1, 2, 3

Serial number

0 indicates the end character

10 means newline

13 means enter

32: space

33 to 47 indicate punctuation

48 to 57 are number symbols

65 to 90 are capital letters

97 to 122 are lowercase letters

127 indicates the delete key

• Keep in mind that

0 is 48; A is 65; A is equal to 97

How to express Chinese

The serial number means China National Bureau of Standards

Name: “GB 2312”

Use 0000 to FFFF to represent Chinese characters

A hexadecimal number has four 0/1 bits

FFFF is 4×4=16 bits, which is two bytes

Max. 2^16 = 65536 characters

But it contains only over 6,000 Chinese characters, Western alphabet and Japanese kana

The GB2312 number of you is C4E3

GB2312 of “cow” is numbered C5A3

Chinese people have rare characters in their names

Li 16 => Li *

How to express the rare characters, traditional characters, Korean, forgot to include the previous

Microsoft stepped in

Microsoft has launched a gb extension, GBK for short, GBK GB extension

Contains 21886 Chinese characters and graphic symbols

It contains almost all Chinese characters used in China, Japan and South Korea

Fully compatible with GB2312

Still using 16 bits (two bytes)

Later, gb launched GB18030 to replace GBK

GB18030 is incompatible with GB2312

How to express other uncoded national languages

Keep numbering and solve the world’s needs at once

Unicode Unicode

• advantages

Has included 130,000 characters (more than 16 bits), universal

It will continue to be expanded and will not stop

The latest version adds only one character – the combination character of reharmony

• disadvantages

Two bytes are not enough; each character needs three or more bytes

It’s not cost-effective to expand all files by 50%

So what?

Using Unicode is lazy when it comes to saving

Utf-8 invented (lowest eight bits)

• The existence of chicken thief
• Utf-8 is a Unicode storage rule, also known as a character encoding rule

Store “A”

The Unicode number of a is 97 and the hexadecimal number is 61

Unicode: 0000000000000000; 01100001

Utf-8 lazy save: 01100001

Three bytes to one byte, than GBK also save

Store “you”

Your Unicode number is 4F60

Unicode: 00000000; 0100111101100000

Figure 111001001011110110100000

It’s still three bytes, no saving, but it saves a little bit on letters

• The 8 in UTF-8 means

At least 8 bits can be used to store one character

The rules of utf-8

• Take “you A” for example

11100100101111011010000001100001

How do I know what character the above represents?

Read 8-bit message 11100100

Three ones are found at the beginning, indicating that the character has three octagons

So read two more 8-bit messages 10111101 10100000

If you don’t want the first 10, you get 0100111101100000

This reverts to your Unicode self:

000000000100111101100000

Read 8 again for message 01100001

Finding that the character starts with 0 indicates that this character is only 8 bits long

This returns the original Unicode A:

000000000000000001100001

If you don’t understand this page, skip it

Just remember the analogy of waiting in line at a restaurant: Ask at the top table (the algorithm was inspired by human activity).

• How does the computer tell the difference between a number 1 and a string 1

Through auxiliary information: such as postfix TXT text is a string, excel table is a number.

JS data type

7 data types (case insensitive)

• Digital number

• String string

• Boolean bool (George Bull is a British mathematician)

• Symbolic symbol

• Empty undefined

• Null null

• Object the object

Conclusion: four bases, two empty objects

The following are not data types

• An array of
• function
• The date of

They both belong to Object

1. Digital number

writing

• Write it as an integer 1
• Written as a decimal 0.1
• Scientific notation 1.23E4
• Octal notation (used sparingly) 0123 or 00123 or 0o123
• The hexadecimal notation is 0x3F or 0x3F
• Binary 0b11 or 0b11

Special values

• Plus 0 and minus 0

They’re all equal to 0. Be careful

• infinity

Infinity 、+Infinity 、-Infinity

• Unexpressible number

NaN (Not a Number) for example, 0/0=NaN

But is it a number (humans currently don’t know what number it is) NaN === NaN =>false

64-bit floating point number

JS The storage form of a number

• Floating point is a floating point, which means the decimal point moves around
• 123.456 can be represented as 1.23456e10^2
• It can also be represented as 12345.6e10^-2
• You don’t usually use fixed point numbers
• The decimal number can always be expressed as any number from 1 to 9, but the binary digit can only be 1. So I don’t have to store the 1. I get rid of it

64-bit stores a number

• Symbol takes 1 bit
• Number 11 in index (-1023~1024)
• Significant digits are 52 digits (the leading 1 is omitted)

For example: 0.5 storage form 12 ^ 1 1/2 = = > symbol: 0 index: 1 valid number: 0 0.625 storage form 0.5 + 0.125 = = = = = = = = = 12 1/2 + 1/8 + 1 * 2 ^ ^ – 1-3 = > binary

Range and accuracy

Range (ignoring sign bits)

• Full exponent 2^1024, full significant digit to get the maximum binary digit
• Number. MAX_VALUE: 1.7976931348623157 e+308
• The negative direction of the exponent is 2^-1023, the significant number is at least 1, to get the minimum value
• Number.MIN_VALUE: 5e-324

Accuracy (significant number)

• A maximum of 52+1 binary digits can represent significant digits
• The decimal equivalent of 2^53 is 9 followed by 15 zeros
• So all 15 significant digits can be represented exactly
• 16-digit significant digits that start with less than 90 are also accurate
• 9110000000000001 will not be saved

2. String String

Two bytes per character (castrated UTF8)

writing

• Single quotes

‘hello’

• Double quotation marks

“Hello”

• Backquotes ` `

hello

• Pay attention to

Quotes are not part of a string, just as book titles are not part of a book title

What if you want to include single quotes inside single quotes? Use escape

escape

Error writing

• ‘it’s ok’
• The JS engine will think ‘it’ is over and will not understand the rest
• Chrome Chicken thief, written illegal

Correct term

• ‘It’s OK’ // This is the escape
• Double quotation marks “It’s OK”
• The quotation marksit's ok

Escape – Use a different way to say what you want (memorization required)

Multiline string

• If you want to enter the string

• When there were no back quotes

Write up very troublesome, can see net path tutorial

String properties

Wait a minute, only objects have properties, why do strings have properties why do strings have properties (they shouldn’t) wait until we get to objects

Length of string

• string.length
• ‘123’.length // 3
• ‘\n\r\t’. Length // 3
• ”.length // 0
• ‘ ‘.length // 1

Read characters by subscript

string[index]

• let s = ‘hello’;
• s[0] // “h”

Note that index starts at 0

• S [0] is the first character

Notice index to length

• let s = ‘hello’;
• S [5] // undefined
• s[4] // ‘o’

Base64 transcoding

• window.btoa

A normal string is converted to a Base64 encoded string

• window.atob

The Base64 encoded string is converted to the original string

• Usually used to hide a resume from a job Posting

E-mail: ZmFuZ3lpbmdoYW5nQGZveG1haWwuY29t

window.atob('ZmFuZ3lpbmdoYW5nQGZveG1haWwuY29t')

Copy the code

=>”[email protected]”

• Sometimes it’s a way to deceive yourself

The so-called “encryption”, also can fool a part of the layman

3. The Boolean Boolean

True or false

There are only two values true and false,

Pay attention to case. It’s all lower case

The following operators yield bool values

• No operation

! value

• Equal to the operation

1 == 2, 1! = 2, 3 == 2, 3 == 3 = = 4

• Comparison operations

1 > 2, 1 >= 2, 3 < 4, 3 <= 4

If with a bool

If statements often need to be checked for true or false

if( value ) { … } else { … }

The question

• It’s fine if value is a bool
• What if value is not a bool? Who’s true? Who’s false
• 1 is true or false, 0 is true or false

0 is false (see five Falsy values)

• ‘1’ is true or false, ‘0’ is true or false

All is true

Five Falsy values of JS (by rote)

• Falsy is a value equal to false but not false
• Undefined null 0 NaN ” (empty string, no Spaces!)
• “And” are not the same thing. Again, be careful!

4. Undefined and NULL are two empty types

empty

Why are there two empty

This is the original (la) creation (JI) of JS

The difference between

There is no essential difference between the default value undefined

• Details a

If a variable is declared but not assigned, the default value is undefined, not null

• Details of the second

If a function does not write return, the default return is undefined, not null

• Details of the three

Front-end programmers, by convention, write null values for non-objects as undefined and null values for objects as null, but only by convention

5. Symbol of symbols

Data types that you don’t use very often

See article zhuanlan.zhihu.com/p/22652486 directly

Summary of the six types (case-insensitive)

undefined null number string bool symbol

These are simple types. Object is called a complex type. We’ll see in the next video

Variable declarations

There are three ways to declare

• Var a = 1 (obsolete declaration)

• Let a = 1 (variable declaration)

• Const a = 1 (constant declaration)

• A = 1 (assign, not declare global variable)

The difference between

• Var is outdated and unusable
• Let is the new, more rational approach
• Const is an assignment that must be declared and cannot be changed
• This last way is wrong. Do not declare it like this

Var variable promotion

We’ll talk about it in the title

Interested can look ahead to the network channel tutorial wangdoc.com/javascript/…

The var statement

Just skip

We write code without var

Explain the topic separately before the interview

Let the statement

The rules

• Follow block scope, that is, scope cannot go beyond {}

{
let b = 1
console.log(b)
}
console.log(b)
Copy the code

• Cannot be repeated

Let a = 1 let a = 2 false

• It may or may not be assigned
• You must declare it before using it; otherwise, an error will be reported
• Globally declared let variables do not become window properties (var changes window properties)
• For loop with LET works wonders (vs VAR)

Const statement

The rules

• Almost the same as let
• There is only one difference: the value must be assigned at declaration, and cannot be changed after assignment

Variable declarations

Specify a value

var a = 1

It also specifies the type (very important)

Var a = 1 number but the value and the type can be arbitrary a = 2 a = ‘string’

The difference between name and ‘name’

The new guy can’t figure it out

Name is a variable (heap exists)

• Variable value, could be ‘name’, could be ‘hello’
• It can be a variable type, either a number or a string

‘name’ is a string constant (in the stack)

Constants are invariants

• ‘name’ must be ‘name’ and cannot be anything else
• The type is a string

Type conversion

number => string

• String(n)

Uppercase String is used to turn something that isn’t a String into a String

N + “most commonly used method: n+ empty string

string => number

• Number (s) of the capital
• parseInt(s) / parseFloat(s)

Turn anything into an integer

The most common method of s-0: s minus zero

• Plus s plus s

x => bool

• Boolean(x)

!!!!! The most common method for x is to take the original Boolean value

x => string

• String(x)

X.tostring () the most common method, suffixed. ToString ()

(1).toString() (1).toString()