JS foundation 4

object

Definition

An unordered collection of attributes , contain attribute and Method , Allow dynamic addition and deletion of attributes

Create objects

1. The constructor is to create a Object New examples , Then add attributes to it

 let person = new Object();
 person.name = 'Nicholas';
 person.age = 29; // Traverse the output  for(var key in person){ 
    
     console.log(person[key])
     }
      // The result is Nicholas  29

2. Literal measure , Add attributes directly to curly braces

let person = { 
    
            name: 'Nicholas',
            age: 29,
        }
        // Traverse the output  for (var key in person) { 
    
            console.log(person[key])
        }
        // The result is Nicholas  29

Object built-in properties

attribute	meaning
Configurable	Is it ok to pass delete Delete and redefine
Enumberable	Is it ok to pass for-in Loop back
Writable	Whether the value of the attribute can be modified
Value	The actual value of the property

To modify the default values of these properties true, Use Object.defineProperty（） Method , This method takes three parameters ： The object to which the attribute is added , Name of property , A description object
Sample code

let person = { 
    
            name: 'Nicholas',
            age: 29,
        }
        Object.defineProperty(person, 'name', { 
    
            writable: true,

        })

        console.log(person.name)
       // result Nicholas
        person.name = 'newNicholas';
        console.log(person.name)
        // writable: true result Nicholas
        // writable: false result newNicholas

String object

establish

1. var str =new string(‘hello’)
2. var str=‘hello’

String properties

attribute	meaning
str[]	Access the characters in the string
charAt()	Find the index number contents and return
indexof()	Find the string index number
lastIndexof()	Find the string index number from the back
substring()	2 It's worth , Where to start , To which end , The ending character is not included
substr()	2 It's worth , Where to start , Intercept a few
slice()	2 It's worth , Where to start , To which end , The ending character is not included
replace()	Replace the specified string with a new string
split()	Delimited string , Returns an array
concat()	String splicing
trim()	Remove the leading and trailing space characters
startsWith(a)	Whether or not to a The beginning of a character
endWith(a)	Whether or not to a Character ending
includes(a)	a Whether the full string exists
toLowerCase	Convert lowercase
toUpperCase	Convert a capital
repeat(N)	Copy the string N Time
padStart(n,‘m’)	take m Copy n Times to fill in front of the string
padEnd(n,‘m’)	take m Copy n Fill the string twice

notes :

1. A space in a string is also a string
2. indexof() Return when you can't find it -1
3. slice,substring,substr

• Is called in the same form , When there is only one number , Extract to end position
• When there is a negative number ,slice Treat all negative values as string lengths plus negative parameter values ,substr Take the first negative parameter value as the string length plus the negative parameter value , The second conversion is 0,substring Convert all to 0
  Sample code

var str='hello';
console.log(str[0])
// result h

var str='hello';
console.log(str.charAt(0))
// result h

var str='hello';
console.log(str.indexOf('l'))
// result 02       

var str='hello';
console.log(str.lastIndexOf('l'))
// result 03

var str='hello';
console.log(str.substring(0,2))
// result he

var str='hello';
console.log(str.substr(0,2))
// result he

var str='hello';
console.log(str.replace('he','eh'))
// result ehllo

var str='hello';
newStr=str.split('')
console.log(newStr)
// result ['h', 'e', 'l', 'l', 'o']

 var str='hello';
 newStr=str.concat(' world')
console.log(newStr)
// result hello world

var str=' hello';
newStr=str.concat(' world')
newStr1=newStr.trim()
console.log(newStr1)
// result hello world

var str='hello';
newStr=str.startsWith('h')
console.log(newStr)
// result true

var str='hello';
newStr=str.toUpperCase()
console.log(newStr)
// result HELLO

for-in And for-of

• foe-in

 var obj={ 
    
            name:'Nichols',
            age:19,
        }
for(var key in obj){ 
    
    console.log(obj[key])
    // result Nichols 19        
        }

• for-of

var str='hello'; for(var value of str){ 
    
     console.log(value)
   // result h e l l 0         
        }

1. for-in Yes, get the key name （ Indexes ）,for-of Is to get the element value
2. for-in You can traverse objects , Array
3. for-of Chronology / Array objects / character string /map/set Such as a collection of iterator objects , But can't traverse objects
4. for-in Traverse the prototype chain for-of Traverse the current object

Template string

Definition ：${ Variable }

1. A string enclosed in inverted quotation marks
2. Template syntax , Output the value of a variable in a string
3. Simple operations in template syntax