Focus on the basic knowledge of JS

Everything_ is_ none 2021-05-04 13:58:32
focus basic knowledge js

A preliminary understanding JaveSctript

  • Web The composition of the standard
title Content explain
structure HTML The structure and content of web page elements
performance CSS The appearance and location of web page elements , Including layout and color 、 size
Behavior js The definition and interaction of web model
  • Javascript Is a programming language , Can be used to create dynamically updated content , Control multimedia , Making image animation and so on to achieve human-computer interaction effect , Simply speaking , You can use short code to achieve magical functions .
  • JavaScript The way of writing , The norm is script The label is written in body above
    • inline JavaScript
    • Inside JavaScript
    • external JavaScript
  • Terminator
    • Represents the end of a statement , In English ; Express .
    • Write but not write .
    • A newline ( enter ) Will be recognized as a Terminator , Therefore, in the actual development, many people advocate writing JavaScript Code , Omit the Terminator .
    • But to unify the style , To finish, write every sentence , Or you don't write every sentence .
  • JavaScript I / O statement
    • Output and access can be understood as the interaction between human and computer , The user uses the keyboard 、 The mouse, etc. inputs information to the computer , The computer processes it and shows it to the user , This is a process of input and output .
    • Output statement
      • ducoment.write(" What to output ")
        • towards body Output content inside
        • If the output is a label , It will also be parsed into web page elements
      • alert( What to output )
        • Page pop-up , Output the corresponding content
      • console.log( What to output )
        • The console outputs the corresponding content , Usually help us debug .
    • Input statement
      • prompt(" Please enter your name ")


  • The goal is : Understand that variables are the basis for computers to store data Containers , To master the variables Declaration method

What is a variable

  • vernacular : A variable is a box .
  • popular : Variables are the containers in which computers store data , It allows computers to remember .

Basic use of variables

How variables are declared

  • You want to use variables , First, create variables ( It's called declaring variables )
  • There are two parts to declaring variables : Declare keywords 、 Variable name ( identification )
  • let That is the keyword ,let( allow 、 The license 、 Give Way 、 want ), The so-called keyword is a special declaration provided by the system ( Definition ) Variable words
  • let grammar
    //let Declare a variable , for example 
    let age
    //let Declare variables and assign values 
    let age = 18
    //let Declare multiple variables at the same time and declare 
    let age = 18, name = " Zhang San "
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  • Use let What should we pay attention to when doing this
    • Allow declaration and assignment to happen at the same time .
    • Duplicate statements are not allowed
    • JavaScript Some built-in keywords in can't be used as variable names
    • Multiple variables can be declared and assigned at the same time
  • Variable assignment
    • After defining a variable , You can initialize it ( assignment ), After the variable name, add a = Number , And then follow the numbers
    • grammar
    let age
    age = 18
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  • Update variables
    • After variable assignment , You can also give it a different value to update it .
    • grammar
    let age
    age = 18
    age = 19
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    • Be careful
      • let Can be reassigned, but cannot be defined repeatedly ( Statement )

The nature of variables

  • Memory : A place in a computer where data is stored , It's like a space .
  • Variable : It's a small space that the program applies for in memory to store data .

Variable naming rules and specifications

The rules : Must abide by the , Failure to comply will lead to mistakes

  • Cannot use keyword .
    • keyword : Characters with special meanings ,JavaScript Some built-in English words . for example :let var if for wait .
  • Only underline 、 Letter 、 Numbers 、$ form , And the number can't start .
  • Letters are strictly case sensitive , Such as Age and age It's a different variable .

standard : Suggest , Failure to comply will not result in an error , But not in line with the general knowledge of the industry

  • The name should be meaningful .
  • Obey hump naming
    • The first character of the first word is lowercase , After that, each word is capitalized . for example userName

data type

  • Everything in the computer world is data .
  • A computer program can process a lot of data , In order to facilitate data management ( Different uses ), Divide the data into different types
data type effect Representation form example
String( character string ) Represents the content of the text Single or double quotation marks in English 'hello'、“world”
Number( Numbers ) Represents digital content Write numbers directly 10,20
Boolean( Boolean ) Two states of opposition , True or false , Right or wrong and so on true and false ture、false
undefined( Undefined ) Means undefined undefined undefined
Object( object ) Represents a common object Key value pairs wrapped in braces {name:nihao}
Null( Null value ) Represents an empty object null null
Array( Array ) Represents an array object Wrap it in brackets [1,2,3]
Regex( Regular ) Express regular objects Wrap it in a backslash /^[+-]?/

Numeric type (Number)

  • The numbers we learn in math , It can be an integer 、 decimal 、 Positive numbers 、 negative 、NaN( The digital ).


  • NaN:not a number; It means a number , But it's a number type .
  • NaN It's not equal to any value , Including myself .


  • Detect whether a value is a significant number , If it's not a significant number, return true, If it's a significant number, return false.
  • Reuse isNaN This method , First, it detects whether the detection value is a number , If not , It's implicitly transformed , use Number() This method first converts the value to a numeric type and then detects it .

Number() conversion

  • Convert string to number , As long as the string contains any invalid numeric character, the result is NAN, An empty string returns 0
  • Convert the reference data type to a number , It's based on toString() Method to string

pareseInt() and parseFloat()

  • parseInt() Convert other data into numeric integers
  • parseFloat() Turn other data into numbers , Take the decimal point .
var a = '12px'
let ary = [13,14]
// It converts other data types first to string types ,ary.toString() The return value is '13,14'.
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String type (String)

  • Through single quotes ''、 Double quotes "" Or back quotes `` Package data is called string , There is essentially no difference between single quotation marks and double quotation marks , Single quotation marks are recommended .
  • A string is made up of zero to multiple characters , Each character has its own index of position , By a length Store the length of the string .
  • Be careful
    • Both single quotation marks and double quotation marks must be used in pairs
    • Single quotation marks / Double quotation marks can be nested inside each other , But you can't nest yourself , Double outside and single inside or single outside and double inside are OK .
    • Escape characters can be used if necessary \ Output single or double quotes

String splicing

ES6 New template string

  • ${} Where is the js expression , It can be variables and variable operations , Ternary operators and so on .
var x = 15
let b = 3
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utilize + Splicing

  • + There is a string on either side of the number , The result is string splicing
  • + There is an object on the left and right sides of the number ( The goal is to turn objects into numbers , Do math )
    • The system first gets the object's [Synbol.toPrimitive] Property value
    • If you don't have this property , Second, get his valueOf(); The original values are all basic data types .
    • If there is no original value yet , Finally, it will be converted to a string toString, It turns out to be string splicing .
let a = 46
console.log(a+' year ')//46 year 
let ary = [12,13]
ary.valueOf()//[12,13] // This is not his original value , All raw values are basic data types 
let num = new Number(10)// This is an object 
console.log(num[Symbol.toPrimitive],num.valueOf) //undefined 10
console.log(num+10) //20
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  • + The number is only on one side , So this side is even strings / object , It's also a number operation

String method

Get the characters in the string

  • Using variables [] You can get
  • utilize charAt() Method
let str = 'fhcsj'
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Convert other data types to string types

  • utilize Variable .toString() Method
  • utilize String( Variable )
  • Be careful :
    • Array to character to , The return value is to separate each item in the array with a comma
    • For ordinary objects, convert to strings , No matter what value is contained in the object , The final return value is "[object Object]"
let num = 15,
ary = [11,13]
let obj = {}
obj.toString()//"[object Object]"
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  • Returns the encoding of the index corresponding to the string


  • String interception


  • Returns a new string , Interception from start The index to the end, But not including end Indexes .


  • Use the specified separator( must ) Parameters , Split a string into arrays ,howmany( Optional ) Define the length of the array , If you don't write howmany Parameters , There is no limit on the length of the array .
    let str = 'fdkfhkds'
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  • use (replacement character ) Replace In the string (substr character ) Or replace a character that matches the regular .
    let str = 'fsfsdhfsdfs'
    console.log(str.replace('fs','i'))//ifsdhfsdfs, It will only replace the first 
    console.log(str.replace(/fs/),i)//ifsdhfsdfs, Writing it like this will only replace the first 
    console.log(str.replace(/fs/g),i)//iidhidi, To write like this is to replace all... In the string fs
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  • indexOf()
    • Returns the string value specified for retrieval (searchValue, Must write ) The index of ,fromIndex( Specify which index value to start with , Optional )
    • If the specified string value is not retrieved in the string , Will return -1.
  • lastIndexOf()
    • Returns the index of the first occurrence of the specified string value from right to left ,fromIndex What character does the rule start with ( This character is counted from the back to the front ). Be careful : its 0 The index is also the first character on the left .
    • If the specified character value cannot be retrieved from the string, return -1
let str = 'fsdhkhds'
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  • Determine whether the string contains the specified character value (searchValue), Starting with the prescribed index (fromIndex), The return value is true perhaps false
let str = 'fdsds'
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Boolean type (Boolean)

  • When expressing positive or negative , In the computer, the corresponding is boolean type , It has two fixed values true perhaps false.
  • Two ways to convert other data types to Boolean types
    • Boolean(value)
    • !!value
  • notes : Only 0、NaN、 An empty string 、undefined The value that becomes Boolean is false, Everything else is true.

Undefined type (undefined)

  • Undefined type is a special type , Only one value undefined.
  • What happens to undefined types ?
    • Only declare variables without assignment , The default value of the variable is undefined, Generally, it is rare to assign a value to a variable undefined.

Symbol( Represents a unique value , It's a basic data type )

let a = Symbol(0)
let b = Symbol(0)
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Object data type

Common object

  • use {} Wrapped key value pairs represent , An array of class 、 example 、 Prototype object ......

Array objects

  • use [] Wrapped expression

Regular objects

  • use // Wrapped expression

Date object

function Function

  • function : Blocks of code designed to perform specific tasks
  • explain : Functions can wrap code blocks with the same or similar logic , Execute the code logic through function calls , The advantage of this is that it helps to simplify the code , Convenient reuse .
  • grammar
    function Function name () {
    The body of the function
    // Function call , The code logic in the function body is executed 
    Function name ()
    // Repeatable , There is no limit to the number of times .
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    • Naming conventions for functions
      • Basically consistent with variable naming
      • Try to use hump naming
      • The prefix is usually a verb
      • Naming suggestions : Common verb convention .
Common verbs meaning
can Determine whether an action can be performed
has Determine whether to mean a value
is Judge whether it is a certain value
get Get a value
set Set a value
load Load some data

Test data type


  • grammar let age = 18;console.log(type age)
  • Be careful
    • typeof Can detect basic data types .
    • typeof undefined The return value of undefined.
    • type null The return value of object.

JS Arithmetic operator

  • Mathematical operators are also called operators , It mainly includes fake 、 reduce 、 ride 、 except 、 Remainder ( modulus ).
describe Symbol
Sum up +
Subtraction -
quadrature *
Seeking quotient /
Take the remainder ( modulus ) %
    • Operators are special
      • + Operators are summation operations in numeric types
      • + Operators are splices in string type .

Data type conversion

Implicit conversion

  • When some operators are executed , The data type is automatically converted inside the system , This type is called implicit conversion .
  • The rules
    • + One of the two sides of the number is a string type , Will automatically convert the other to a string .
    • except + All arithmetic operators other than the symbol convert data to numeric types .

Explicit conversion

  • It's not rigorous to rely too much on implicit transformation inside the system when writing programs , It's better to use explicit conversion .
  • Number( data )
    • Turn to number type
    • If there are non numbers in the string content , When the conversion fails, the result is NaN(Not a Number) It's not a number .
    • NaN It's also number Data of type , For non numeric
  • Boolean( data )
    • To Boolean
    • 0、 An empty string 、NaN、undefined、null Turn into false, Everything else is true.
  • Variable .toString()

ES6 Knowledge point

Deconstruct assignment

  • ES6 Deconstruction assignment in , Mainly for objects and arrays , The left side defines a structure similar to the right side value , Declare a few variables like this , Quick access to every part of the information .

Array structure assignment

let arr = [1, 2, 3, 4]
let [x, y] = arr
console.log(x, y)//1, 2
// If you use it below x,y Variable structure assignment , May be an error , say x,y Already exists 
let[y,...x] = arr// Report errors 
//... Represents the residue operator , It means that the rest of the array is taken and put into b in 
let [a,...b] = arr
console.log(a, b)//1, [2, 3, 4]
// The comma stands for each of the preceding items 
let[, , , x, y] = arr
console.log(x, y) // 40 undefined
// You can also give a default value to the variable assigned to the structure , If there is no value, it is equal to the default value 
let [, , , , b = 0] = arr
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Object's deconstruction assignment

// Declared by default " Variable " Need and " Consistent property names ", In this way, the object can get the value of the specified member 
let obj = {
name:' Everest training ',
let {name,age} = obj
console.log(name, age) // ' Everest training ' 11
// Make a statement x Variable , And will Assign a value to x
let {name: x} = obj
// When deconstructing assignment , You can set default values for variables 
let { x = 0 } = obj
// If this property value does not exist , Then the default value is assigned 0
// And the number property name , We can rename a new variable to receive the value 
// If you write it directly, you will report an error , There are two ways to report an error 
let {0} = obj
let {[0]} = obj
// Write it correctly 
let{0: x} = obj
// Get the domain name and address of the website quickly 
let {hostname:domain, pathname:path} = location;
// The combination of array structure assignment and object structure assignment 
let ary = [100,' Hello ',{name: 'hello',score:[12, 13]}]
let [, y, {score:[, math]}]
console.log(y,math)//' Hello ',13
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