JS Variable declaration Promotion

First, scope

In JS, the scope is the area within the function, called the functional scope.

Second, variable declaration

Before ES6, a variable was declared through Var, but after ES6, two more keywords were added to declare the variable: letand const

var: declares a variable that is scoped to the current execution context

Let: declares a local variable of a block-level field, and the variable it declares is valid only within the code block in which it resides

Const: A constant that declares a read-only block-level field, and the variable it declares is valid only within the code block in which it resides

1{//code block2   varA = 1;3Let B = 2;4Const C = 3;5   6Console.log (a);//17Console.log (b);//28Console.log (c);//39 }Ten  OneConsole.log (a);//1 AConsole.log (b);//error, REFERENCEERROR:B is not defined (...) -Console.log (c);//did not execute, because the above statement error, so this statement is no longer executed, if the above statement does not error, then here will be an error

1(function (){2   varA = 1;3Let B = 2;4Const C = 3;5   6Console.log (a);//17Console.log (b);//28Console.log (c);//39})();//for convenience, the self-executing function is used hereTen  OneConsole.log (a);//error, REFERENCEERROR:A is not defined (...) AConsole.log (b);//not executed -Console.log (c);//not executed

Let and const do not, like Var, can cause variable elevation phenomena

1 console.log (a);    // error, REFERENCEERROR:A is not defined 2 Let a = 2; 3 Console.log (a);    // Pending Execution

ES6 explicitly states that if a let and a const command exists in a chunk, the variables declared by the block on those commands form a closed scope from the outset. If you use these variables before the declaration, you will get an error.

Third, variable declaration promotion

JS performs a full parsing (including local variables) of the declaration portion of the entire script file before execution, thus determining the scope of the variable.

When a local variable has the same name as a global variable, the scope of the local variable overrides the scope of the global variable. When you leave the scope of a local variable, it goes back to the scope of the global variable.

1             varA=1;2             functionTest () {3alert (a);//undefined here A is not a global variable, the function has already declared a local variable name, so the global variable will be overwritten, JS execution before the declaration part of the resolution, so here's a only declared but not assigned value4A=4; Here A is assigned a value5alert (a);//4, where a is still a local variable6                 varA; Local variable A is declared here7alert (a);//48             }9 test ();Tenalert (a);//1, where A is a global variable

1     vara=10;2         functionTest () {3a=100;4alert (a);//100 Here A is a local variable, if there is no third row a=100, here is undefined, with the third row, where the local variable is assigned a value of5Alert This. a);//10 The inside of the function refers to the function caller, where the function is called by the global object, so this is a pointer to the global object (this refers to window)6             varA;7alert (a);//100/local variable8         }9Test ()

in basic statements (or blocks of code) (e.g.,,, if语句 , for语句 while语句 switch语句 , for...in语句 etc.), there is no promotion of variable declarations

Iv. function Declaration Promotion

function declarations and function expressions are syntactically equivalent, but a little different, the function declaration is referred to the top of the scope, and the function expression does not.

1Fun ();//Hello2 3 functionFun () {4Console.log ("Hello");5 }6 7 // --------------8 //after promotion9 Ten functionFun () { OneConsole.log ("Hello"); A } -  -Fun ();//Hello

1Fun ();//error, Typeerror:fun is not a function2 3 varFun =function(){4Console.log ("Hello");5 };6 7 // --------------8 //after promotion9 Ten varFun ; One  AFun ();//error, Typeerror:fun is not a function -  -Fun =function(){ theConsole.log ("Hello"); -};

JS Variable declaration Promotion