First knowledge of JavaScript (day06) data types

In the computer, the storage space occupied by different data is not invested. In order to divide the data into data with different memory sizes and make full use of the storage space, different data types are defined.

Simply put, the data type is the category and model of data. For example, the name "Zhang San" and age 18 are different.

1.2 data types of variables

Variables are used to store values. They have names and data types. The data type of variables determines how to store bits representing these values in the memory of the computer. JavaScript is a weak type or dynamic language, which means that the type of variables does not need to be declared in advance. During the operation of the program, the type will be determined automatically.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    // int num = 10;  java
    //var num; //   We are not sure which data type the num here belongs to
    var num = 10 ; //num input numeric
    //The variable data type of JS is determined only during operation according to the value to the right of the equal sign
    var str ='Zhang San'; //str string model

</script>
</body>
</html>

JavaScript has dynamic types, which also means that the same variables can be used as different types

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
  //js is a dynamic language, and the data types of variables can be changed
    var x = 10; //x is numeric
    x = 'Zhang San'; //x is a string
</script>
</body>
</html>

1.3 classification of data types

JS divides data types into two categories

Simple data type (Number, String, Boolean, Undefined, null)

Complex data type (Object)

Simple data types in JavaScript and their expressions are as follows:

Simple data type	explain	Default value
Number	Numeric type, including integer and floating-point values, such as 21 and 0.21	0
Boolean	Boolean value types, such as true and false, are equivalent to 1 and 0	false
String	String type, such as "Zhang San" Note that in JS, strings are quoted	""
Undefined	var a； The variable a is declared but not assigned. In this case, a=undefined	undefined
Null	var a = null declares that variable a is null	null

2 digital type

2.1 digital hexadecimal

The most common base systems are binary, octal, decimal and hexadecimal.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    var num = 10;   //num numeric
    var PI = 3.14;  // PI digital
    //1. Octal 0 ~ 7. Add 0 in front of the number in our program to represent octal
    var num1 = 010;
    console.log(num1) // 010 octal to hexadecimal is 8
    var num2 = 012;
    console.log(num2);
    //2. The hexadecimal 0 ~ 9 a~f#ffffff digits are preceded by 0x, indicating hexadecimal
    var num3 = 0x9;
    console.log(num3);
    var num4 = 0xa;
    console.log(num4);
</script>
</body>
</html>

At this stage, we only need to remember to add 0 before octal and 0x before hexadecimal in JS

2.2 digital range (for understanding)

Maximum and minimum values of values in JavaScript

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
   alert(Number.MAX_VALUE);//1.7976931348623157e+308
   alert(Number.MIN_VALUE);//5e-324

</script>
</body>
</html>

2.3 three special values of digital type

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    alert(Infinity);//Represents infinity, greater than any value
    alert(-Infinity);//Represents infinitesimal, less than any value
    alert(NaN);       //Nan not a number represents a non numeric value
</script>
</body>
</html>

2.4 isNaN()

It is used to judge whether a variable is of numeric type and returns true or false

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    //isNaN() this method is used to judge non numbers and return a value. If it is a number, it returns false,
    // Returns true if it is not a number
    console.log(isNaN(12)); //false
    console.log(isNaN('Zhang San'));//true

</script>
</body>
</html>

3. String type string

String type can make any text in No. 1, and its syntax is double quotation mark "" and single quotation mark ''

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    var myname = "Zhang San";  //Use double quotation marks to represent strings
    var myname1 = 'Li Si'; //Use single quotation marks to represent strings
    var myname = Wang Wu;     // If an error is reported and no one is used, it will be considered as JS code, but JS does not have these syntax
        
</script>
</body>
</html>

Because the attributes in HTML tags use double quotation marks, we prefer to use single quotation marks in JS.

3.1 string quotation mark nesting

JS can nest double quotation marks with single quotation marks, or nest single quotation marks with double quotation marks (outer double inner single, outer single inner double)

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
   var StrMsg = 'I am"tall, rich and handsome"Cheng xuape';//Can '' contain ''
   var StrMsg = "I am'tall, rich and handsome'Cheng xuape";//Can contain ''
    //error
   var StrMsg = "I am"tall, rich and handsome"Cheng xuape";//error
</script>
</body>
</html>

3.2 string escape character

Similar to the special characters in HTML, there are also special characters in the string, which we call escape characters

Escape characters start with \. Common escape characters and their descriptions are as follows:

Escape character	interpretative statement
\n	Newline, n means newline
\\	Slash\
\'	'single quote
\"	Double quotation mark
\t	tab indent
\b	b means blank

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    //String escape characters start with \, but these escape characters are written in quotation marks
   var Str = "I am'tall, rich and handsome'of\n Cheng xuape";//Can contain ''

</script>
</body>
</html>

3.3 string length

The string is composed of Soft Liver characters. The number of these characters is the length of the string. The length of the whole string can be obtained through the length attribute of the string.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
   //1. Detect the length of the obtained string
    var str  = 'my name is andy';
   console.log(str.length);  //15
</script>
</body>
</html>

3.4 string splicing

Multiple strings can be spliced with + in the form of string + any type = new string after splicing

Before splicing, any type added to the string will be converted into a string, and a new string will be spliced.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
   //1. Detect the length of the obtained string
    var str  = 'my name is tony';
   console.log(str.length);  //15
    //2. String splicing
   console.log('Zhang San'+'Li Si');//String, Zhang Sanli Si
   console.log('Wang Wu'+18);//Wang wu18
   console.log('Lei Longjian'+true);//Thunder Dragon Sword
   console.log(12 + 12)//24
   console.log('12'+12)//1212
</script>
</body>
</html>

+No. summary formula: add values and connect characters

3.5 string splicing reinforcement

We often splice strings and variables, because variables can easily modify their values

Variables cannot be quoted because quoted variables will become strings

If there is string splicing on both sides of the variable, the formula "quote, add", delete the number, and unload the variable and add the middle

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    console.log('Zhang San'+18);//Zhang San 18
    console.log('Zhang San'+18+'year')//Zhang San is 18 years old
    var age = 18;
    //We don't write variables into strings. We implement them by connecting them with strings
    console.log('Zhang San'+age+'year');//Zhang San is 18 years old
</script>
</body>
</html>

Topics: Java Front-end

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