2022.7.22 JS entry common data types and methods

One 、script label

1. script Label writing

<!-- type You don't have to write , If you want to write, write it right  -->
<script type="text/javascript"></script>

<!--  here type Deliberately write wrong in order to get inside  h1  To replace the above to render  -->
<script type="text/tpl">
    <h1>{
   { name }}</h1>
</script>

2.script Tags can be externally referenced

Two 、Number

1. Declaration definition ：

	// Literal declaration 
    var num = 1;
	console.log(typeof num);

    // Digital object mode declaration 
	var num = new Number(1);
	console.log(num+4);

2. Digital method

Integer judgment

        var num = 10

        console.log(Number.isInteger(num)) //true

Specify the number of decimal places to return

        var num = 5.2641654156

        console.log(num.toFixed(2)) // Returns two decimal places , The result is 5.26

3.NaN Indicates a non numeric value ,isNaN()  Judge whether it is a number

	var num = '5'
	console.log(Number(num))
	console.log(isNaN(num))
    // The output is  5  and false

        null and '''' （ empty ） Will be judged as a number

4.js There will be errors in floating-point number calculation

	var a = 0.1+0.2
	console.log(a)
	// The result is 0.30000000000000004. The solution is  toFixed()  Method 
	var b = 0.1+0.2
	console.log(b.toFixed(1))
	// The result is  0.3

3、 ... and 、String Common methods

// concatenation operator 
var address = "xdclass.net",
name = " Xiaodi classroom ";
console.log(name + " The website is ：" + address);

// To obtain the length of the 
console.log("xdclass.net".length)

// toggle case 
console.log('xdclass.net'.toUpperCase()); //XDCLASS.NET
console.log('XDCLASS.NET'.toLowerCase()); //xdclass.net

// Remove the blank 
var str = '   xd class.net  ';
console.log(str.length);
console.log(str.trim().length);

// Get single character 
console.log('xdclass'.charAt(3)) //l
console.log('xdclass'[3]) //l

// Intercepting string 
var n = 'xdclass'.slice(1,5);
console.log(n); //dcla
slice(start, end) //start（ contain ）,end（ It doesn't contain ）

// Find string 
console.log('xdclass.net'.indexOf('s')); //5
console.log('xdclass.net'.indexOf('s', 6)); //6  From 6 Start searching with two characters （ Including 6 individual ）

// Replace string 
var name = " Xiaodi classroom ";
var changeName = name.replace(" Classroom ", " educational services ");
console.log(changeName); // Xiaodi educational institution 

// Type conversion ( take "2022-01-22" The time format is converted to "2022/01/22")
var name = '2022-01-22';
console.log(name.split('-')); //['2022', '01', '22']

var a = name.split('-');
console.log(a.join('/')); //2022/01/22

         In type conversion ,split It takes a certain value as an interval to convert a string into an array ,join Then convert the interval point to the required value and return to the string form . In the above example '-' If replace with '' The following results will be produced

Four 、Boolean usage

1. Implicit conversion

// Almost all types can be implicitly converted to Boolean type

// Almost all types can be implicitly converted to  Boolean  type             
true            false
String      Non empty string          An empty string 
Number      Not 0 The numerical          0/NaN
Array       When the array does not participate in the comparison     Empty array for comparison 
Object       
undefined                   
null                         
NaN                         

         When other types are related to Boolean When comparing types , Other types will be converted to numeric types first and then compared .[] Converting an empty array to a number is 0 ,[1, 2] Non empty yes NaN .undefined and null The conversion to a number is NaN.

 2. Explicit conversion
 

var xd = '' ;

console.log(!!xd); //false

xd = 0;

console.log(!!xd); //false

xd = null;

console.log(!!xd); //false

One !  Is to reverse the original value and convert it into Boolean , Two !! Is the original value Boolean value .

5、 ... and 、  Basic type data -Undefined、Null、Symbol

1.Undefined/Null These two are basically synonymous

undefined == null  //true

//null Is a representation of “ nothing ” The object of , The conversion to a value is 0 ;undefined Is a representation of “ nothing ” Original value , When it is converted to a number, it is NaN.

    console.log(Number(null)) //0

    console.log(Number(undefined)) //NaN

    console.log(Number(1+null)) //1

    console.log(Number(1+undefined)) //NaN

2.Undefined

// Variables are declared but not assigned , Is equal to undefined .

var i ; // i undefined

// Object has no assigned property , The value is undefined .

var a = Object(); //a.name undefined

// Function has no return value default return undefined

var x = f(); //x undefined

3.Null

null Express “ There is no object ”, That is, there should be no value .

usage ：

        （1） As an argument to a function , The parameter representing the function is not an object

        （2） As the end of the object prototype chain

                Object.getPrototypeOf(Object.prototype) // null

4.Symbol（es6 The grammar of ） Generate unique attributes

// You need to add the same attribute name to an object

var xd = { aa: '1' };

var aa = Symbol('aa');

xd[aa] = '2';

console.log(xd);