Write blog at leisure ~ briefly talk about let, VaR and Const

Catalog

1. Variable

1.1 Repeated statement

1.2 Scope

1.3 Variable Promotion

2. Constant

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1. Variable

let and var Is a data storage container that declares variable types

What is a variable ？ Variable The saved value is variable. The saved parameter value can be changed and re assigned according to the needs , such as

var num1 = 1;

console.log(num1); // 1

num1 = num1 + 1;

console.log(num1); // 2

let num2 = 1;

console.log(num2); // 1

num2 = num2 + 1;

console.log(num2); // 2

Are these all right ?

This is within the scope of normal system operation

Then it's all variable declarations. What's the difference between them ?

1.1 Repeated statement

var a = 1; // a = 1

a = 2; // a = 2

//  Normal operation, right ? No problem, right ? I change its internal value 

var a = 1; // a =1

// Then when I continued to write the code, I forgot that I stated a 了 , I made it clear again a~ At this time ~ The system says no problem ! You declare that I will store it ! Old fellow iron is not defective.

let a = 1; // a = 1

a = 2; // a = 2

let a = 1; // Uncaught SyntaxError: Identifier 'a' has already been declared

You see you see Why is this wrong ? Identifier declared ? This is the legendary statement that cannot be repeated ~let Declared variable containers cannot be declared repeatedly or ~ You see, I was reminded ?let This variable will be protected during and after declaration Prevent parameter confusion in repeated declarations, that is ~

var a = 1; // a = 1

let a = 1; //  no way 



let a = 1; // a = 1
var a = 1; //  Not good either. !!!!

1.2 Scope

stay js In the code

let a = true
if(a) {
    var j = a;
    let i = a;
}
console.log(j) // true
console.log(i) // i is not defined

The same execution declares the output ! Why is the result so different ? stay js in es6 Declarative grammar let It is also called block level scope declaration , Brace scope declaration . all if for function In the sentence let Declarations cannot be read externally ! and var stay if for After the declaration in, the external can still read , Only function in var Variables of will have the effect of scope isolation ! 

1.3 Variable Promotion

What is the promotion of variables ? As the name suggests, the promotion of variables is to promote variables !

console.log(a); // a is not defined
//  Output an undefined variable   It will prompt that it is undefined ! This is the detection function in the system to remind us that there is a problem in this place ! Buddy   Change it ?

var b;
console.log(b); // undefined
// undefined A variable that declares that it is not copied will be defined as undefined! Why not not defined? Because this variable is declared ,a is not defined It's a false report undefined It's output b Value   It's just b The value of is undefined only ~ Let's go on 


console.log(c); // undefined
var c = 0;
//  This ! It's stated   Also assigned   Why is his result still  // undefined ?  At least he should give me a not defined!! Why is it like this ? This involves the promotion of variables !
//  stay js When the code is executing , The system will read the code first .  When reading the variable declaration, the declaration will be promoted to the head . That is to say, this variable has been declared at the time of output ~ Just write in the back var c = 0; This one is just assigned 


console.log(c); // undefined
var c = 0;
//  In fact, it is equivalent to 

var c;
console.log(c); // undefined
c = 0;
//  In this way !  Just look at it like this ?


//  and let Would have been different 
// console.log(d); // Cannot access 'd' before initialization
let d = 0;
console.log(d); // 0
//  Ah ~ by the way   I just don't want to be promoted !  The system read me ? Go to school ~ Let me improve ? I don't do it ~ Now I declare   Then I need to use it after that ~ I just listen to the programmer's father and stay here ~ So the system told you   You use this before initialization ! You have problems with these ~ Check the block ~ Um. ~ That's it ~ By the way ~ When an error is reported, the following console.log Can't output ~

 

We often state

2. Constant

consot Is a data storage container for declaring constant types

What is a constant ？ constant Immutable value It is often used in the immortal world …… Set the number ！！

const num3 = 1;

console.log(num3); // 1

num3 = num3  + 1;

console.log(num3); // Uncaught TypeError: Assignment to constant variable.

It's said to be a fixed number You still want to change it ~ My fate depends on me, right ? How to know whether it's OK without trying ? Now I'm wrong ? Constants cannot be reassigned ! The value stored inside it is immutable But available Computable It is a common read-only type in computers Value can read it Don't try to change it ~

Don't think about it ?emm It seems that how to say ~

  const num4 = {a: 1};

  console.log(num4.a); // 1

  num4.a = 2;

  console.log(num4.a); // 2

How has it changed ? It doesn't mean you can't become ?

This is because {a:1} Is the reference data type What this data type stores in variables is actually the address value corresponding to the data stored in stack memory , We are changing one of these address values a Attribute time constant num4 The saved address value is ' unchanged ' Of Become just in the object a Value . This may be a small potential that can be changed , Don't go against the general trend ~ The system only pays attention to the physical address value stored in constants during operation And for changes in address values It is not concerned ! So as long as the constant is not

const num4 = {a: 1};

num4 = {a: 1};

This operation of repeated assignment is ok You say he is still equal to the original object ~ No, there is only one  {a: 1} Do you ? sorry Even if the value inside is exactly the same It is also two different objects in the computer ! This involves reference data types, basic data types and stack storage emmm Write it in the following article