Updated from time to time, only for notes

 

catalogue

1, Comparison of var, let and const

2, Cookie, localStorage, sessionStorage comparison

3, String connection

1. Use connector "+"

2. join method using array

3. Use the template string backquote ` 'of ES6

4. Using concat method

4, JS operator

1. Non air operator

2,?? Non air operator

3,??= Null assignment operator

4,?. Chain judgment operator

5,? : Ternary operator

1, Comparison of var, let and const

	var	let	const
Scope	Global scope, function scope, and {} will also be falsely called block scope	Block level scope	Block level scope
Variable promotion	Yes, but you will get the default value undefined	No, ReferenceError will be prompted before declaration, which is called "temporary dead zone" (TDZ for short)	No, ReferenceError will be prompted before declaration, which is called "temporary dead zone" (TDZ for short)
Multiple declarations	sure	It is not allowed to declare the same variable repeatedly within the same scope	It is not allowed to declare the same variable repeatedly within the same scope
Global object	Global variables declared with var and function are still the attributes of global objects. Global variables declared with var cannot be deleted from window/global (Global is for node environment) with delete	A global variable declared with the const command is not a property of a global object	A global variable declared with the const command is not a property of a global object
variability	sure	sure	Cannot be assigned again
initialization	Not required	Not required	essential

2, Cookie, localStorage, sessionStorage comparison

	cookie	localStorage	sessionStorage
life cycle	It is generally generated by the server, and the expiration time can be set. If a Cookie is generated on the browser side, it will expire after closing the browser by default	Save permanently unless cleared	Valid only when starting a session, cleared after closing the page or browser
data size	About 4K	Generally 5MB
Communication with server	It will be carried in the HTTP header every time. If you use cookies to save multiple data, it will cause performance problems	It is only saved in the customer service terminal and does not participate in the communication with the server
Ease of use	The native Cookie interface is unfriendly and needs to be encapsulated by itself	The native interface is acceptable, and can also be encapsulated again to better support Object and Array

3, String connection

1. Use connector "+"

let a = 'java'
let b = a + 'script'  // javascript

2. join method using array

let arr = ['hello', 'java', 'script']
let str = arr.join("")  // hellojavascript

3. Use the template string backquote ` 'of ES6

let a = 'java'
let b = `hello ${a}script`  // hello javascript

4. Using concat method

var a = 'java'
var b = 'script'
var str = a.concat(b)  // javascript

4, JS operator

1. Non air operator

function (obj) {
  let a = obj || {}
}

Equivalent to

function (obj) {
    let a;
    if (obj === 0 || obj === '' || obj === false || 
        obj === null || obj === undefined) {

        a = {}
    } else {
        a = obj
    }
}

2,?? Non air operator

function (obj) {
  let a = obj ?? {}
}

Equivalent to

function (obj) {
    let a;
    if (obj === null || obj === undefined) {
        a = {}
    } else {
        a = obj
    }
}

3,??= Null assignment operator

This assignment operator assigns a value only if the value is null or undefined

let x = null
let y = 5
console.log(x ??= y)   // 5

4,?. Chain judgment operator

Allows you to read property values deeply nested in the object chain without having to validate each reference. When the reference is empty, the expression stops evaluating and returns undefined

let travelPlans = {
  destination: 'DC',
  monday: {
  location: 'National Mall,
  budget: 200
}
}

console.log(travelPlans.tuesday?.location)  // undefined

5,? : Ternary operator

function checkCharge(charge) {
  return (charge > 0) ? 'yes' : 'no'
}

console.log(checkCharge(20))  // yes