Five simple data types of JS

js Have dynamic types , It also means that the same variables can be used as different types , But here we only introduce js Five simple data types for , For complex data types (object) No introduction .

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String( String type )

Definition

var str = ' character string ';

1. String type can be any text in quotation marks , Its grammar is Double quotes “” and Single quotation marks ’’
2. because HTML The attributes in the tag use double quotation marks ,JS Here we recommend using single quotation marks .

String quotes nested JS

You can use single quotes to nest double quotes , Or use double quotes to nest single quotes ( Double outside and single inside , Single outside and double inside )

var strMsg = ' I am a " Gao Shuai Fu " Program the ape '; //  It can be used '' contain ""
var strMsg2 = " I am a ' Gao Shuai Fu ' Program the ape "; //  It can also be used. ""  contain ''
//  Common mistakes 
var badQuotes = 'What on earth?"; //  Report errors , You can't   Single and double quotation marks 

String escape character

similar HTML The special characters in it , There are also special characters in strings , We call it the escape character .
All the escape characters are \ At the beginning , The common escape characters and their descriptions are as follows ：

String length

A string is made up of several characters , The number of these characters is the length of the string . Through strings length Property to get the entire character The length of the string .

var strMsg = " I'm a handsome and golden procedural ape ！";
alert(strMsg.length); //  Show  11

String splicing

1 You can use... Between multiple strings + Splicing , The splicing method is character string + Any kind of = The new string after splicing
2 Any type added to the string will be converted into a string before splicing , And then splice it into a new string

//1.1  character string  " Add up "
alert('hello' + ' ' + 'world'); // hello world
//1.2  Numeric strings  " Add up "
alert('100' + '100'); // 100100
//1.3  Numeric strings  +  The number 
alert('11' + 12); // 1112

1 We often splice strings and variables , Because variables can easily modify the values inside 2 Variables cannot be quoted , Because quoted variables become strings
3 If you have string concatenation on both sides of the variable , formula “ Lead and add ”, Cut out the numbers , Variable write and middle

console.log('pink teacher ' + 18); //  As long as there are characters, they will be connected 
var age = 18;
// console.log('pink teacher age Years old! '); //  That's not going to work 
console.log('pink teacher ' + age); // pink teacher 18
console.log('pink teacher ' + age + ' Years old! '); // pink teacher 18 Years old! 

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Number( Digital )

JavaScript Numeric types can be used to hold integer values , You can also save decimals ( Floating point numbers ）.

var age = 21; //  Integers 
var Age = 21.3747; //  decimal 

Digital base system

The most common system is binary 、 octal 、 Decimal system 、 Hexadecimal . At this stage, we just need to remember , stay JS Add... Before the octal 0, Hexadecimal is preceded by 0x

 // 1. Octal number sequence range ：0~7
var num1 = 07; //  Corresponding to the decimal 7
var num2 = 019; //  Corresponding to the decimal 19
var num3 = 08; //  Corresponding to the decimal 8
 // 2. Range of hexadecimal number sequence ：0~9 as well as A~F
var num = 0xA; 

Digital range

JavaScript The maximum and minimum of the values in
Maximum ：Number.MAX_VALUE, The value of 1.7976931348623157e+308
minimum value ：Number.MIN_VALUE, The value of ：5e-32

alert(Number.MAX_VALUE); // 1.7976931348623157e+308
alert(Number.MIN_VALUE); // 5e-324

Three special values of digital type

Infinity , For infinity , Greater than any number
-Infinity , For infinitesimal , Less than any number
NaN ,Not a number, Represents a non numerical value

alert(Infinity); // Infinity
alert(-Infinity); // -Infinity
alert(NaN); // NaN

isNaN()

Used to determine whether a variable is a non numeric type , return true perhaps false

var usrAge = 21;
var isOk = isNaN(userAge);
console.log(isNum); // false ,21  It's not a non number 
var usrName = "andy";
console.log(isNaN(userName)); // true ,"andy" It's a non number 

Boolean( Boolean type )

The boolean type has two values ：true and false , among true Said really （ Yes ）, and false Said the false （ wrong ）. When Boolean and number are added ,
true The value of is 1 ,false The value of is 0.

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

Undefined （ Data type not defined ）

A variable that is not assigned after declaration has a default value undefined ( If you join or add , Pay attention to the results ）

var variable;
console.log(variable); // undefined
console.log(' Hello ' + variable); //  Hello undefined
console.log(11 + variable); // NaN
console.log(true + variable); // NaN

Null（ Null value ）

A declaration variable is given to null value , The value stored in it is empty （ When learning objects , We continue to study null)

var vari = null;
console.log(' Hello ' + vari); //  Hello null
console.log(11 + vari); // 11
console.log(true + vari); // 1