JS knowledge points

javascript Is a weakly typed scripting language that runs on Web pages

Weak type ： That is, you do not need to specify the data type when declaring variables , Decide when assigning

Scripting language ： Small volume , Incomplete programming language

cmd,（ Suffix is .bat The file is also a scripting language ）

Three ways of writing

① Directly in html Write in Tags

<input type="button" value=" Button " οnclick="alert('Hello js')">

② stay body It's written inside script middle

<script>

           alert("Hello js");

</script>

stay head It's written inside script, To write a load function

<script>

          window.onload = function(){

                alert("hello js");

}

</script>

③ Lead to the outside js file

 <script src="../js/03-js"></script>

external js Also write the loading function

js Component part

1.js The core grammar of

2.DOM operation （ use js operation HTML and CSS）

3.BOM operation （ use js Operating the browser ）

js The core grammar of

Variable ： The amount of data that changes

           The essence of a variable is a container , There is a data inside

Declare variables ：var Variable name = value

        The variable name should not be in Chinese , Do not start variable names with numbers , When you choose a variable name, you need to know its meaning

data type

① Basic data type

1.number（ The number ） 2.string（ character string ） 3.boolean（ Boolean type ） 4.undefined( Undefined ) 5.null（ Assigned but empty ）

② Reference data type

object,array,function,date, Custom object

Operator

Arithmetic operator （+,-,*,/,%,++,--,** yes x Of y Power ）

Comparison operator (>,<,>=,<=,==,!=,===)

Logical operators ：&,|,&&,||

One 、 Branching structure

①if Single branch statements

②if..else Two branch statements

③if..else if...else Multi branch statement

Two 、 Loop statement

for loop

for（ Initialization statement ; Judgment statement ; Iteration statement ）{

                Execute statement ;

}

while loop

Initialization statement ;

while( Judgment statement ){

                Execute statement ;

                Iteration statement ;

}

do...while loop

Initialization statement

do{

        Execute statement ;

        Iteration statement ;

}while（ Judge the condition ）

Mathematical statement

math.floor() Rounding down

math.ceil() Rounding up

math.round() rounding

math.random() Take random number

continue Out of the loop , And back there perform

break Out of the loop , Don't follow

Dead cycle ：while（true）{

                }

                for（;;）

&nbsp; Small space &emsp Big space （ amount to 2.5 Small spaces ）

DOM operation ：

DOM:JS Code changes html Label object （ Property value , Text value ）

1. obtain html Label element object

1. Assign a value to an object's property , Change attribute value , Get attribute value （classname）

Yes html Tag's own properties , You can call directly with the element object css attribute , Put it in style Integration properties , therefore ： Element object .style. attribute

bg.style.positionX=x+'px'

Two ways to delete

① Element delete children

ele.remove();

② Delete the child element through the parent element

fu,removeChild(zi)

parentNode: Get parent node

parentElement: Get parent element

this Represents the current object （ Method call ）

Under function call ,this Point to window

es6 New characteristics ：

const Constant

let Variable