Numbers and Strings

  • 1 is not the same as 1

Different types of storage

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

How to Store numbers

Decimal to binary

  • 31 (decimal) = 01111 (binary)
  • I’m going to do it to the power of two

Binary to decimal

  • Two to the NTH power

Binary is represented in hexadecimal

  • Remember 8, 4, 2, 1 for X, X, X, X
  • Every four digits from left to right is one digit
  • HEX indicates hexadecimal and BIN indicates binary
  • OCT means base 8, DEC means base 10

Image credit: Hungry Man Valley

How to store characters

Denoted by numbers

  • 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

How to express Chinese

  • National Bureau of Standards of China No. : National Standard 2312 (16 bits two bytes)
  • Microsoft has pulled out of a national standard extension called GBK.

Unicode

  • advantages
    • Has included 130,000 characters (more than 16 bits), universal
    • It will continue to be expanded and will not stop
  • disadvantages
    • Two bytes is not enough; each character needs three or more bytes
    • All files expanded by 50%

JS data type

7 species (no. 8, BigInt)

  • Digital number
  • String string
  • Boolean bool
  • Symbolic symbol
  • Empty undefined
  • Null null
  • Object the object
  • Conclusion: four bases, two empty objects

The following are not data types

  • Arrays, functions, dates
  • They both belong to Object

Boolean value

  • true or false
  • 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(value){… }else{… }
  • Pay attention to
    • It’s fine if value is a bool
    • If value is not a bool
    • All except falsy and false are true
  • Five falsy value
    • undefined
    • null
    • 0
    • NaN
    • ‘ ‘

Undefined and null are two empty types

  • There is no essential difference:
  • If a variable is not assigned, the default value is undefined, not null
  • If a function does not write return, the default return is undefined, not null
  • Traditionally, null values for non-objects are written as undefined and null values for objects are written as null

Number (number: 64-bit floating point number)

  • Integer written
    • 1
  • The decimal notation
    • 0.1
  • Scientific enumeration
    • 1.23 e4
  • Octal notation (used sparingly)
    • 0123 or 00123 or 0O123
  • Hexadecimal notation
    • 0 x3f x3f or 0
  • Binary notation
    • 0 bl1 or 0 bl1

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)

64-bit floating point number

  • A form of digital storage
    • 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
  • 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)

Range and accuracy

  • Range (ignoring sign bits)
    • Exponential and significant digits are pulled to get the maximum binary number
    • Number. MAX_VALUE: 1.7976931348623157 e+308
    • The negative direction of the index is full, the significant number is at least 1, and the minimum value is obtained
    • 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

String string

writing

  • Single quotes
    • ‘hello’
  • Double quotation marks
    • “Hello”
  • The quotation marks
    • hello

escape

  • Write it another way to say what you want
    • \ ‘said’
    • \ “said”
    • \n indicates a newline
    • \r means enter
    • \t represents the TAB character
    • \ \ \
    • \uFFFF indicates the corresponding Unicode character
    • \xFF represents the first 256 Unicode characters

String properties

  • Length of string
    • ‘123’.length
    • ‘\n\r\t’.length
    • ”.length
    • ‘ ‘.length
  • Read characters by subscript
    • string[index]
    let s = 'hello';
s[0]  //"h"
Copy the code
  • Note that index starts at 0
  • Notice index to length
    let s = 'hello'
    s[5]  // undefined
    s[4]  // 'o'
Copy the code

symbol

Not really, you know

Variable declarations

There are three ways to declare

  • var a = 1
  • let a = 1
  • const a = 1
  • a = 1

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 approach is wrong and cannot be declared as such

Var variable promotion

Let the statement

  • The rules
    • Follow block scope, that is, use scope beyond {}
    • Cannot duplicate declaration
    • 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
    • The for loop works wonders with let

Const statement

  • The rules
    • Almost the same as let
    • There is only one rule: the value must be assigned at declaration time and cannot be changed after assignment

Variable declarations

  • Specify a value
    • var a = 1
  • The type is also specified
    • var a = 1
  • But both the value and the type can vary at will
    • a = 2
    • A = string

Type conversion

number => string

  • String(n)
  • n + ”

string => number

  • Number(s)
  • parseInt(s) / parseFloat(s)
  • s – 0
  • +s

x => bool

  • Boolean(x)
  • !!!!! x

x => string

  • String(x)
  • x.toString()

BigInt

  • Is a built-in object that represents greater than2^53 - 1The integer
  • Cannot be used forMathMethods in objects
  • Not with anyoneNumberInstance mixed operation, both must be converted to the same type, butBigIntintoNumberYou lose accuracy

Just a quick note and I’ll fill it in later