JS precompile some small knowledge

Do you remember the original native JS from vue and React? Ashamed of my today began to review the original knowledge, but also incidentally some notes, for later often recall, notes are watching the teaching video copy, if there is a mistake, welcome to correct

O factorial

function nums(a){

            if(a === 1 || a === 0) {return 1;
                
            }
            return a*nums(a- 1)}console.log(nums(5))  / / 120
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Fibonacci numbers

    function nums(a){
                    if(a === 1) {return ;
                    }
                    return  nums(a- 1)+nums(a2 -)}console.log(nums(5))
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precompiled

JS execution process

1. Grammatical analysis

2. The precompiled

3. Explain execution

Ii. The precompilation process

1. Simple rules:

Function declaration, overall promotion

Variable declaration, declaration of promotion

2. Compilation process

1. Create an AO object

2. Find the parameter and variable declarations and replace the parameter and parameter names with the AO attribute name value bit undefined

3. Unify the parameter values and parameters

4. Find the function declaration inside the function body and assign the value to the function body

Example 3.

 function a(a){
          console.log(a)  // function

          var a = 123

          console.log(a)  / / 123

          function a(){}

          console.log(a)  / / 123

          var b = function (){}
          console.log(b)  //function
           
       }
       a(1)

// Perform step analysis

//1. Create AO objects

AO {

}

//2. Find the parameter and variable declarations and replace the variable and parameter names with the AO attribute name value bit undefined
Ao {
    a: undefined.b: undefined
}

//3. Unify the argument values and parameters
Ao {
    a: 1 , // a is first assigned to the value 1 of the calling parameter
    b: undefined   // Function B is not promoted because function b is a function expression and only promotes variables
}

// 4. Find the function declaration in the function body, the value assigned to the function body
Ao {
    a: 1 -> function a(){}, // function a(){
    b: undefined   // Function B is not promoted because function b is a function expression and only promotes variables
}
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Example 4.

function test (a, b){
          console.log(a) // The first 1

          c = 0
          var c;
          a = 3;
          b = 2;
          console.log(b) // The second 2

          function b(){}
          function d(){} 
          console.log(b) // The third 2
      }
      test(1)
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Example 5.

      function test (a, b){
         console.log(a)  // function a(){}
         console.log(b) // undefined
         var b = 234
         console.log(b)  / / 234
         a = 123
         console.log(a)  / / 123
         function a(){} 
         var a;
         b = 234;
         var b = function(){}
         console.log(a)  / / 123
         console.log(b)  // function(){}
      }
      test(1)
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6. Sample

      global = 100;
      function fn(){
          console.log(global) // undefined
          global = 200
          console.log(global) / / 200
          var global = 300
      }

      fn()
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Example 7.

        function test(){
            console.log(b) //undefined
            if(a){
                var b = 100
            }
            c = 123

            console.log(b) //undefined
        }
        var a;
         test();
         a = 10;
         console.log(c) / / 123
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8. Implicit conversion

console.log(typeof(a)) // "undefined"
console.log(typeof(null)) //object
/ /!!!!!" " => true
/ /!!!!!" " => false
 if(typeof(a)&& -true + (+undefined) +"") {/** * typeof(a) 'undefined' * -true -1 * +undefined NAN * -true + (+undefined)+"" "NAN" **/
        // So the entire IF statement is true
          console.log(1)}Copy the code

closure

But any function inside is saved outside, forming a closure

        function a(){
            var aa = 0
            function b(){
                aa++
                console.log(aa)
            }
            return b;
        }
        // a()() 1
        // a()() 1
        let demo = a()
            demo() / / 1
            demo() / / 2
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    function test(){
        var num = 100
        function a(){
            num++
            console.log(num) / / 101
        }
        function b(){
            num--
            console.log(num) / / 100
        }
        return [a,b]
    }

    var mtArr = test()
    mtArr[0]()
    mtArr[1] ()Copy the code

Execute function immediately

(function(){
    var a= 123
    var b= 456
    console.log(a+b)
}())

+function(){} ()var test = function(){} ()// This cannot be executed and an error is reported
function(){

}()

// This cannot be executed, but no error is reported
function(a,b,c){},1.2.3)
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function test (){
            var arr = []
            for(var i=0; i<10; i++){ arr[i] =function(){
                    console.log(i)
                }
            }
            return arr;
        }

        var myArr = test()
            myArr[0] ()/ / 10
            myArr[1] ()/ / 10


/ / rewrite
        function test (){
            var arr = []
            
            for(var i=0; i<10; i++){ (function(i){
                    arr[i] = function(){
                        console.log(i)
                    }
                }(i)) 
            }
            return arr;
        }

        var myArr = test()
            myArr[0] ()/ / 0
            myArr[1] ()/ / 1
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