JavaScript scripting language - usage details of all basic data types (comprehensive)

notes

Single line comment ctrl + / (VSCode)

Multiline comment Shift + alt + a(VSCode)

/**/

There are keyboard shortcut settings in the lower right corner of VSCode - > Enter default shortcut - > double click to change

JS input / output statement

method	explain	ascription
alert(msg)	Browser pop-up alert box	browser
console.log(msg)	Browser console printout information	browser
prompt(info)	The browser pops up an input box, which can be entered by the user	browser

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
            //This is an input box
            prompt('Please enter your age');
            //Alert pop up alert box
            alert('Age input is correct');
            // Console console output to programmer for testing
            console.log('Press F12，console What can be seen in');

    </script>
</head>
<body>

</body>
</html>

variable

Declare variable

var age;	//Declare a variable named age

About var

var is a JS keyword, which is the word (the abbreviation of variable).
After using this keyword to declare a variable, the computer will automatically allocate memory space > for the variable, which does not need to be managed by the programmer.
age is the variable name defined by the programmer. We need to access the allocated space in memory through the variable name.

assignment

age = 10;	//Consistent with java

Output variable

<script>
        var myname = "Taotao";
        var age = 12;
        console.log(myname);
        console.log(age);
</script>

prompt gets the variable

<script>
    var name = prompt('Please enter your name' + name);
    alert('Your name is' + name);
</script>

Declare multiple variables at the same time

<script>
    var name = 'Liu Hongtao',age = 10,address = 'train station';
</script>

Variable defaults

var age;

If var is not assigned as undefined

Direct use without declaration

console.log(age);

age not defind

Do not declare direct assignment (not recommended)

age = 100;
console.loog(age);  //Can be used, but not recommended

Variable name specification

Like Java, it is no longer described

Java identifier, package name and naming specification (emphasis)_ Ghost Knight blog - CSDN blog

data type

Data type classification

Simple data type

Number
String
Boolean
Undefined

var a;
//Variable a is declared but no value is given. At this time, a = undefined

Null

var a = null;
//Declared that variable a is null

Complex data type

object

Numeric Number

Numeric type in hexadecimal representation

<script>
    //Octal, preceded by "0"
    var num1 = 010; 
    console.log(num1);  //8

    //Hexadecimal, preceded by "0x"
    var num2 = 0x12
    console.log(num2);  //18
    
</script>

Numeric maximum value and minimum value

<script>

console.log(Number.MAX_VALUE);
console.log(Number.MIN_VALUE);
</script>

Infinity

Infinity

console.log(Number.MAX_VALUE * 2);

Infinitesimal

-Infinity

console.log(Number.MIN_VALUE * 2);

Non numerical

console.log('Taotao' - 100);

Judgment numerical isNaN() method

If it is a number, it returns false; otherwise, it returns true

<script>
var a = 1;
var b = isNaN(a);
console.log(b);
</script>

//false

String string (single quotation mark is recommended)

Single and double quotation marks are supported

String nesting

var info = 'I am"Taotao"';
console.log(info);

String wrap

Similar to java, it will not be described

Get string length

var info = 'I am"Taotao"';
console.log(info.length);

String splicing "+"

var name = 'Hong';
console.log('Liu' + name + 'Waves');

The details are similar to java and will not be described

Boolean type

true

The default value is 1

console.log(true + 1);
//2

false

The default value is 0

console.log(false + 1);
//1

Difference between Undefined type and Null type

A variable that has not been assigned a value after declaration will have a default value of undefined (pay attention to the result when connecting or adding)

var variable = undefined;
console.log(variable + 'pink');	
//undefinepink
console.log(variable + 1);
//NaN

var space = null;
console.log(space + 'pink');
//Nullpink
console.log(space + 1);
//

Get data type method

typeof

console.log(typeof 1);
//number
console.log(typeof null);
//object

typeof details

var age = prompt('Please enter your age');
console.log(age);
//18
console.log(typeof age);
//string

Console color and type

number in blue

Black is string

Light blue is boolean

Literal

Literal is the representation of a fixed value in the source code
Generally speaking, literal quantity indicates how to express this value

Numeric type conversion

Convert to string type

mode	explain	case
toString()	Convert to string	var num = 1;alert(num.toString());
String() cast	Convert to string	var num = 1;alert(String(num));
Plus concatenated string	And string splicing results are strings	var num = 1;alert(num + "I am a string");

Convert to digital (key)

mode	explain	case
parseInt (string) function	Convert string type to integer numeric type	parseInt('78')
parseFloat(string) function	Convert string type to floating-point numeric type	parseFloat('78.21')
Number() cast function	Convert string type to numeric type	Number('12')
js implicit conversion (- * /)	Implicit conversion to numerical type by arithmetic operation	'12' - 0

details

console.log(parseInt('3.9'));
//3
console.log(parseInt('120px'));
//120
console.log(parseInt('r120px'));
//NaN

Implicit conversion

console.log('12' - 0);
//12
console.log('122' - '120');
//2
console.log('122' * 1);
//122

Convert to Boolean

Boolean() function

console.log(Boolean('true'));
//true
console.log(Boolean(0));
//false

Keywords: Javascript Front-end Visual Studio Code

Added by rblais666 on Thu, 13 Jan 2022 18:39:03 +0200

Programming VIP