1. Data type and type conversion

(1) Data type and typeof()

The following basic data types are provided in JavaScript

String string

Number value

Boolean     Boolean type

null     empty

undefined   Undefined

typeof (dat) can get the data type of the specified data

(2) Type conversion

1. Convert to numeric type

There are three fixed syntax for converting data to numeric types

Number (DAT) converts dat data to numeric values

parseInt (DAT) converts dat data to an integer

Parsefloat (DAT) converts dat data to floating point numbers

console.log(Number(5),typeof(Number(5)));
console.log(Number(3.14),typeof(Number(3.14)));
console.log(Number("12"),typeof(Number("12")));
console.log(Number("zhangsan"),typeof(Number("zhangsan")));
console.log(Number("12px"),typeof(Number("12px")));
console.log(Number("a12px"),typeof(Number("a12px")));
console.log(Number(false),typeof(Number(false)));
console.log(Number(true),typeof(Number(true)));

 

  2. Convert to string

There are three syntax for converting data to strings

String(dat)   Convert dat data to string data

dat.toString()   Convert dat data to string form

dat + “”     Convert dat to string form using concatenated string form

console.log(String("zhangsan"),typeof(String("zhangsan")));
console.log(String(1223),typeof(String(1223)));
console.log(String(false),typeof(String(false)));
console.log("zhangsan".toString(),typeof("zhangsan".toString()));
console.log((123).toString(),typeof((123).toString()));
console.log((false).toString(),typeof((false).toString()));
console.log("zhangsan"+"",typeof("zhangsan"+""));
console.log(123+"",typeof(123+""));
console.log(false+"",typeof(false+""));

'  

3. Convert to boolean type

  The fixed syntax for converting data to Boolean types is Boolean (dat)

console.log(Boolean(0),typeof(Boolean(0)));
console.log(Boolean(0.0),typeof(Boolean(0.0)));
console.log(Boolean(-20),typeof(Boolean(-20)));
console.log(Boolean(""),typeof(Boolean("")));
console.log(Boolean(" "),typeof(Boolean(" ")));
console.log(Boolean("abc"),typeof(Boolean("abc")));

 

  4.null and undefined conversion (PIT)

Convert to numeric type (note)

console.log(Number(null),typeof(Number(null)));
console.log(parseInt(null),typeof(parseInt(null)));
console.log(parseFloat(null),typeof(parseFloat(null)));
console.log(Number(undefined),typeof(Number(undefined)));
console.log(parseInt(undefined),typeof(parseInt(undefined)));
console.log(parseFloat(undefined),typeof(parseFloat(undefined)));

Convert to string

console.log(String(null),typeof(String(null)));
console.log(null+"",typeof(null+""));
console.log(String(undefined),typeof(String(undefined)));
console.log(undefined+"",typeof(undefined+""));

 

console.log((null).toString(),typeof((null).toString())); //An error will be reported here
console.log((undefined).toString(),typeof((undefined).toString())); //An error will be reported here

  Convert to Boolean

console.log(Boolean(null),typeof(Boolean(null)));
console.log(Boolean(undefined),typeof(Boolean(undefined)));

(3) Type implicit conversion

When some code is executed, it will automatically complete the data type conversion at the bottom. This conversion method is implicit conversion

There are two common implicit conversions  

1.isNaN(dat)

Function: judge whether a data is a non number

console.log(isNaN("123"));
console.log(isNaN("123a"));

 

2.dat.toFixed(n)

Function: convert data of a numeric type into a string after specifying the number of decimal places to be reserved

console.log((3.141592654).toFixed(2),typeof((3.141592654).toFixed(2)));

 

  2. Operator

(1) Common operators

1. Arithmetic operator

Symbol	describe	Example
+	Addition operator	12+9
-	Subtraction operator	12-9
*	Multiplication operator	12*9
/	division operator	12/9
%	Remainder operator	12%9
---,++	Self decreasing and self increasing operators	a++，a--

2. Assignment operator

Symbol	describe	Example
=	Assign values to variables	var a = 12
-=	Extended subtraction	var a-=12 => var a=a-12
+=	Extended addition operation	var a+=12 => var a=a+12
*=	Extended multiplication	var a*=12 => var a =a*12
/=	Extended Division	var a/ = 12  => var a=a/12

3. Relational operators

Symbol	describe	Example
>	Greater than relation	18 > 16 the result returns a Boolean value
<	Less than relation	18<16
>=	Greater than or equal to	18.<=16
<=	Less than or equal to	18 <=16
==	Equal value	10 = "10"
!=	Values are not equal	10 != '11'
===	Value and type are equal [identity]	10 === '10'
!==	Values or types are not equal	10 !== '10'

4. Logical operators

Symbol	describe	Example
&&	Judge whether both sides are true (logical and)	10 <12 && 21 <22
||	Judge whether the conditions on both sides are false (logical or)	10<12 || 12>>13
!	Direct negation of results (logical non)	!(10 > 12)

(2) Use of operators

1. Use of arithmetic operators

var a = 12;
var b = 9;
console.log(a+b,a-b,a*b,a/b,a%b,a++,b--);

  Note: a + + means operation before self addition, and + + a means self addition before operation

var a =10;
var b = 20;
var c = a++ + ++a +b++;
/*
 *Analytic c= 10 + 12 + b++
 *      c=  10 + 12 + 20
 */
console.log(a,b,c);

2. Use of comparison operators

console.log(12>18,12<18,12>=18,12<=18,12=="12",12==="12",12!="12",12!=="12");

 

In addition to numbers, you can also compare strings: you compare the ascii code of strings  

console.log("1000">"100","a">"b","ab" > "b","ab">"aaa");

  3. Use of logical operators

console.log(10>9&&10>8,10>9&&10<8,10<9&&10<8);
console.log(10>9||10>8,10>9||10<8,10<9||10<8);
console.log(!(false));