ghostwu simple and easy to understand javascript variable promotion

1 a = 'ghostwu';2 var a;3 console.log( a );

For the above example, you may think that the output result of the third line of code should be undefined, because the second line is var a; which declares the variable but does not assign a value, so a The value is undefined, but the correct result is ghostwu. As for why, please continue reading!

1 console.log( a );2 var a = 'ghostwu';

For the above example, the first line of code, you may think An error is reported because the a variable is not defined before a is output, but the correct result is undefined. Well, it seems a bit inexplicable.

To understand why, we must first clarify the following two points:

• javascript code is not executed line by line.

• javascript execution is divided into 2 steps:

• Compilation (lexical interpretation/pre-interpretation)

• Execution

Secondly, when we encounter var a = "ghostwu" to define a variable, js actually regards this sentence as a two-stage thing, var a occurs in In the compilation phase, a = 'ghostwu' occurs in the execution phase. Then var a will be promoted to the front of the current scope, a = 'ghostwu' stays in place waiting for the execution phase, so:

1 a = 'ghostwu';2 var a;3 console.log( a );4 5 //上面这段代码经过编译之后，变成下面这样6 7 var a;  //被提升到当前作用域的最前面8 a = 'ghostwu'; //留在原地，等待执行9 console.log( a );

1 console.log( a ); 
2 var a = 'ghostwu';3 4 //上面这段代码,经过编译之后，变成下面这样5 6 var a;7 console.log( a );8 a = 'ghostwu';

After reading the compiled code, do you understand?

Before going on to the following, let us first clarify the two common ways of defining functions:

1         //函数声明, 形如:2         function show(){3             console.log( '函数声明方式' );4         }5 6         //函数表达式, 形如:7         var show = function(){8             console.log( '表达式方式' );9         }

Because expressions and function declarations, during the compilation phase, will produce different interpretation effects.

1         show();2         function show(){3             console.log( a );4             var a = 'ghostwu';5         }

How to explain the above code during the compilation phase? Just remember the following sentence:

The function declaration will be promoted

So, after the above code is compiled, it becomes the following:

       function show(){    //函数声明被提升到 当前作用域的最前面
            var a;    //var声明被提升到当前作用域的最前面, 注意，他不会提升到函数的外面, 因为当前的作用域是在函数中            console.log( a );
            a = 'ghostwu';
        }
        show();

So, the above result is undefined;

For function expressions, they will not be promoted. See the following example:

 1 show(); //报错,show is not a function 2 var show = function(){ 3  console.log( 'ghostwu' ); 4 } 5 //对于上面这段表达式代码，经过编译之后: 6 var show; 7 show();  //执行之后就是 undefined(), 所以在表达式定义之前，调用函数报错了 8 show = function(){ 9   console.log( 'ghostwu' );  
10 }

1         show(); //你好2         var show;3         function show(){4             console.log( '你好' );5         }6         show = function(){7             console.log( 'hello' );8         }

Why is the result of the above code "Hello"?

Because: When a function declaration with the same name appears, When variables are declared, function declarations will be promoted first and variable declarations will be ignored. So after compilation, it becomes:

1         function show(){2             console.log( '你好' );3         }4         show(); //你好5         show = function(){6             console.log( 'hello' );7         }8         show();//如果这里在调用一次，就是hello, 因为show函数体在执行阶段 被 重新赋值了

If there is a function declaration with the same name, the later one will overwrite the previous one, as follows:

         show();                                console.log( 'hello'          show =              console.log( '你好'                       console.log( 'how are you!'                        console.log( 'how are you!'          show();          show =              console.log( '你好'          show();

 1 //思考题： 请问下面的结果是什么？ 为什么？ 写下你的答案 2          show(); 3           var a = true; 4           if( a ){ 5               function show(){ 6                   console.log( 1 ); 7               } 8           }else { 9               function show(){10                  console.log( 2 );11             }12          }

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