1. JavaScript variable promotion mechanism

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1. In order to make the code execute from top to bottom , First, it will open up a piece of memory （ Stack memory ）=> Scope / Execution context context

2. Variable declarations （ promote ）

When the browser opens up memory , And immediately execute the code from top to bottom , Instead, do the following operations first ： Put all bands in the current scope var/function Keywords are declared and defined in advance => Variable promotion mechanism

 2.1  belt var We only declare in advance that （declare）, No assignment , The default value is undefined.
 2.2  belt function Advance declaration , And define （defined）,“a=13” Definition is assignment , To be precise , Is to associate a variable with a value .
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console.log(a); //undefined
var a=12;
var b=a;
b=13;
console.log(a); //12
console.log(b); //13
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console.log(sum(10.20)); //30
function sum(num1,num2){
    return num1+num2
}
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3. The difference between function expression and function declaration ：

4. stay if In the judgment condition （ Global scope ）：

4.1  Variable declaration is the same regardless of whether the condition is successful or not , Variable promotion is required ;
4.2  Under judgment conditions , Functions have special properties , Older browsers declare or define in advance ; In the new version , For compatibility es6 Grammatical norms of , You can only promote variables in advance , Cannot define in advance 
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console.log(fn); //undefined
// fn();  //=>fn is not function;
if('fn' in window){
    fn();  //=> Ha ha ha ;
    function fn(){
        console.log(" Ha ha ha ");
    }
};
fn(); // Ha ha ha 
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5. The self executing function itself does not carry out variable promotion ( No name )

 Self executing functions ： Add in front () 、！、+、-、~, There is only one purpose , According to the grammar .


  
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