2.13 weekly report

Built-in objects

Js The language comes with , For developers , Provide necessary functions （Math Array String Date）

Math object （ Not a constructor ）：Math.PI PI Math.ceil() Rounding up , That is, take the larger value

 Math.round() rounding （.5 special , Take it from the big one ）    Math.abs() The absolute value Math.floor() Rounding down , That is, take the small value   

Math.random() Returns a random decimal number [0,1）

A random number between two numbers , And include these two numbers

Date object （Date）: If there are no parameters , Returns the current time of the system

Common writing of parameters     Digital 2019,10,01   String type ‘2019-10-1 8:8:8’

Format date

Get the total number of milliseconds （ distance 1970.1.1）

Countdown cases ：

Variable attribute differences

identical ： Are used to store data

Different ： Variable declaration and assignment Write the variable name directly Exist alone

      Attribute in object There is no need to declare When used, it is object . attribute

The difference between function and method

identical ： To achieve a function Do something

Different ： Functions are declared separately Call function name （） Exist alone Method is inside the object Different calls

character string

1. String immutable

The inner value remains unchanged , Changing the content is actually changing the address , Memory opens up a memory space

1. Return position according to character

str.indexOf(‘ Characters to find ’, The starting position )

1. Returns the character according to the position

charAt(index)  str.charAt(index) Return character

charCodeAt(index) str.charCodeAt(index) Return character's ASCII code

str[index] Compatibility

1. Splice and intercept strings

Splicing ： var str= ‘Andy’;

       str.concat(‘red’)

Intercept ：str.substr(‘ Intercept start position ’,’ Intercept a few characters ’)

1. Replace string Convert to array （185）

Replace ：str.replace(‘a’,‘b’) hold a Replace with b    Just replace one

Convert to array ：split(‘ Separator ’) String separator str.split(‘ Separator ’)

1. Case study （183 184）

data type

Simple data type （ Basic data type , Value type ）

string number Boolean undefined null( For an empty object )

Stack ： Opening up space directly puts value

Complex data type （ Reference type ）： adopt new Keyword created objects （Object Array Date）

First store the address on the stack （ Hexadecimal ） Then point to the data in the heap

Web APIs

Page interaction function

API：（ Application programming interface ） Some predefined functions , Help us realize some function

Web API: It is a set of operating browser functions and page elements provided by the browser API