ES6 learning path (II) let & const

let

except var Now you can use let Declare variables

let The declared variable has a block level scope

    {
    
      let a = 1
      var b = 2
    }
    console.log(a) // ReferenceError: a is not defined
    console.log(b) //  The above error line of code did not run , Normally, it can be printed 2 Of 

Blocks do not affect each other , If the following n use var Definition , that n Namely 3 了

   (() => {
    
      let n = 5
      if (true) {
    
        let n = 3
      }
      console.log(n) // 5
    })();

Duplicate declaration of variables... Is not allowed

   let i = 0
   let i = 1 // Uncaught SyntaxError: Identifier 'i' has already been declared

for Circulation and let

for Inside the loop body is a child scope

    for (let i = 0; i < 2; ++i) {
    
      let i = 'a'
      console.log(i) // a It was printed 2 Time , Because the same scope variable cannot be declared repeatedly , So inside the loop i It's different 
    }

No variable promotion

    console.log(foo) // undefined
    var foo = 'a'
    console.log(bar) // Uncaught ReferenceError: Cannot access 'bar' before initialization
    let bar = 'b'

use let Declared variables , Can only be defined before using ,var Variables defined , The script exists when it runs . It seems js It was designed very casually , Ha ha ha . When beginners learn java When , Remember that variables must declare types , I never thought that variables could be used before they were declared .

Temporary dead zone

One sentence summary : use let and const Declared variables , Remember to define first , After use , Otherwise, it will report a mistake .

Interesting knowledge

    var foo = 'a';
    (() => {
    
      console.log(foo)
      if (false) {
    
        var foo = 'b'
      }
    })();

The console prints undefined, My personal understanding is , although if Statement not executed , But when the script runs ,if Statement internal var foo Is running , That is to say foo This variable exists , And covered with an outer layer , It's just that there's no assignment . No assignment , Didn't the representative program go in .

miscellaneous

• Try to avoid declaring functions in a block level scope
• Block level scopes must have braces
• Remember to declare before you use

const

const It is usually used to declare constants , That is, once this variable is declared , The value cannot be changed . Objects and arrays , Except for functions , Because objects, arrays and functions belong to reference data types . For example, I define an object obj, const obj = {}, Upper figure .obj What is actually stored is a pointer , Point to obj The location of this object in the heap . So for obj Adding and deleting attributes is ok Of , But if you obj = {}, It's going to be a mistake , Because you modified pointer Pointer pointing .

    const a = 1
    console.log(++a) // Uncaught TypeError: Assignment to constant variable.

Same as let There is a temporary deadband , Block level scope , Variables also need to be defined first , After use .

Top objects

Code up

    var a = 1
    console.log(window.a) // 1
    let b = 2
    console.log(window.b) // undefined

In the browser environment , Top level objects refer to window, stay node The middle finger is global. The assignment of top-level objects is the same as that of global variables .