JS--Scope problem

<title>js– scope problems</title> js– scope issues Table of Contents

• Variable declarations and function declarations

function F () {    a = 3;                       equivalent to WINDOW.A = 3}f (); alert (a);                        3functionF() {    vara = 3;} f (); alert (a);                        Not defined

var t = 10; function F () {    3

var t = 10; function F () {    vartTen

var a = 3; function F1 () {    a = 4;    function F2 () {        a = 5;    }    F2 ();} F1 ();d ocument.write (a);       5

var a = 3; function F1 () {    vara = 4;    function F2 () {        a = 5;    }    F2 ();} F1 ();d ocument.write (a);       3

Variable declarations and function declarations

For the parsing process of the declaration: Parameter –> variable –> function declaration

Rule 1:
For variable declarations
If there are arguments and declared variable names that are the same, they cannot be overwritten (no re-declaration of a variable), but there are assignments that override, and rules 1 and 2 only discuss variable declarations.

Rule 2:
for function declarations
Function declarations can override variable declarations and parameters

--For rule 1 function F (a) {    vara;                       Alert (a) is not declared here    ;                    8}f (8); function F (a) {    vara = 2;    alert (a);                    2}f (3);

// --For rule 2function F() {var a; alert (a);function a() {} alert (a);} f ();// outputs two a functions, stating that the original a declaration is overwritten by a function declarationfunction F() {var a=' Hello ';// At the end of the assignment, function A is overwritten with ' hello ', and the assignment is parsed after the variable declaration and function declaration analysis.alert (a);function a() {} alert (a);} f ();// output of two ' Hello 'function a(b) {alert (b);// First, assign a value of 1b =function() {// there is no function declaration, direct assignment, and then assign a value to a functionalert (b); } b ();} A (1);// The result is 1 and a function

JS--Scope problem