I. JS Variable Declaration, Data Type and Variable Conversion

1.js variable declaration keyword: var

Note: a: js variables are case sensitive;

In b: js, double quotation marks or single quotation marks can be used for strings.

The same name variable can be declared in c: js. The console will not report errors, but the later variable will override the previous one.

2. Classification of data types

a: number (number type)

b: string (string type)

c: boolean (boolean type)

    d: null

e: undefined (undefined)

f: object (object type)

Note: NaN is a digital type, but its essence is not a number; it is not equal to all values, including itself.

3. Conversion of JS variables

a: Convert the data type to the digital type using number () function

b: Use Boolean () function to convert data type to boolean type

Note the use of js variables:

Variable names are best defined, starting with letters or underscores, followed by numeric underscores.

a: var age= 12；
b: var test5 ='hello';
c: var _test = 'hello';

2. Three ways of introducing js and checking whether browser javascript supports or not

1. Internal introduction

<script>
......
</script> 

2. External introduction (recommended)

<script src="Route" type="text/javascript"></script>

3. Intra-line introduction

<a href="javascript:confirm('aaa');">666</a>
<p onclick="javascript:alert('hello');">clickme</p>

4. Check whether the browser supports javascript usage (generally supported by computers)

<noscript>
Your browser does not support JavaScript
</noscript>

3. Hump Marking and Underline and js

a. Hump marking and underscoring

var firstName = 'king';    //Small hump
var FirstName = 'queen';    //Great Hump
var first_name = 'fyh';    //Underline method

Three kinds of bullet-frame modes of b and js

Warning frame: alert (');

Confirmation box: confirm('');

Tip box: prompt('prompt statement','input box');

Operators in js

1. Arithmetic operators (+,-,*,/,+,--,%)

<script>
    var i=1,j=2;
    document.write(i+j);  //3
    document.write('<br/>');
    document.write(i-j);   //-1
    document.write('<br/>');
    document.write(i*j);  //2
    document.write('<br/>');
    document.write(i/j);  //0.5
   
     document.write(i%j);  //2
    document.write('<br/>');
     document.write(1%-2);  //1
    document.write('<br/>');
     document.write(-1%2);  //-1
    document.write('<br/>');
     document.write(-2%1);  //0
</script>

a:"%" remainder operator, divide the two numbers and take the remainder.

　　　　a=10%2; //a=0

b:"+" self-incremental operator

var a=10;
    //Add 1 after assignment.
var b=a++;
    //The value of a variable is assigned to b, b=10, and then a+1, a=11.
var c=++a;
    //The value of variable a is + 1, a=12, and then variable a is assigned to variable c, c=12.
document.write("b="+b+",c="+c);

c:"- -" self-decreasing operator

　　var num1=2;

alert(- -num1); output num1=1;

// Floating Point Supports Self-Increasing and Self-Decreasing Operators

　　　　num1=12.3；

alert (- num1); / / output num1=11.3;

Note: String types do not support self-increasing and self-decreasing operators

2. Assignment operators (=, +=, -=, *=, /=)

"+=": First add and then wait, such as a+=5, expand and then a=a+5;

"-=": First subtract, then wait, such as a-=5, expand and then a=a-5;

"*=": First multiply, then wait, such as a*=5, then expand to a=a*5;

"/=": First divide and then wait, such as a/=5, expand and then a=a/5;

3. String operator (+)

String operators are used only as connectors, and no other operations are performed.

  eg: var a='fyh';

　　  var b=a+'666';　　　　//b='fyh'+'666'='fyh666';

4. Comparing operators (>,<,>=,<=,<=,=,! = ====,! = =)

　　eg: a=5>6;   //a=false;

　　　　a=5>=6;  //a=false;

"==", value comparison, value return true, otherwise return flase;

"===", value and type are compared at the same time, the same return true, different return flase;

"! == "Not absolutely equal, one value or type is not equal, or both are not equal;

"===", absolutely equal, equal in value and type;

5. Logical Operators (&, |,!)

&& (and): Return true only when both sides are established, and false if they are different;

|| (or):a: the first is ture, and the second is false or ture, which returns the previous value.

b: The front is false, and the back, whether it's ture or false, returns the back value.

6. Comma Operator

var a,b,c;
var n=1,m=2;
var z=(x=3,y=4);
console.log(z);//4
console.log(x);//3

7. Ternary Operator (Trinomial Operator) (a > b?'true':'false')

if(3>1){
        document.write('aa');
}else{
        document.weite('bb');
}

document.write('<br/>');
var res=3>1?'aa':'bb';
document.write(res);
document.write('<br/>');

8. void operator

<script type="text/javascript">
    //void operator
    z=void(n=1,m=2,p=3);
    alert(z);
            //z Output is 3
        var x;
    x=123;
    x='king';
    x=true;
    x=null;
    x=undefined;
    x=[1,2,3];
    alert(typeof x);
        //x The output is undefined    
</script>    

V. Comparison of null, NaN, "" and undefined

Undefined: undefined type

　　　　var a;

alert(a); //undefined not defined;

2. null (null value)

　　　　alert(typeof null);   //object

3. NaN(NaN is a digital type, but its essence is not a number; it is not equal to all values, including itself;)

　　　　alert(NaN==NaN);//false;

　　4,　" "

0==" ";   //true

0==false; //true　

null==undefined;   //true 

0==undefined； //false

undefined==false; //false

" "==undefined； //false

0==null； //false

null==false; //false

" "==null； //false