Global scope and function scope in js

Scope:

Scope refers to the scope of the role of a variable. There are two scopes in the street:

Function scope exists within a function, and global variables can be used throughout the program.The relationship between the two scopes is shown in the following figure:

As shown in the figure above, both function scope 1 and function scope 2 are in the global scope.And the two scopes of function scope 1 and function scope 2 are not related to each other, and the variables in each independent function scope do not affect each other.If there is an undefined variable in the function scope, it will be looked for in the global scope.

Global scope:

Global scope:- The js code written directly in the script tag is in the global scope- The global scope is created when the page is opened and destroyed when the page is closed- There is a global object window in the global scope, which represents a browser window,It is created by the browser and we can all use it directly.- In the global scope: the created variables are saved as properties of the window object.The created function will be saved as a method of the window object.- Variables in the global scope are global variables that can be accessed from any part of the page.Early declaration of variables: Variables declared with the var keyword will be declared (but not assigned) before all code is executed.But if the variable is declared without the var keyword, the variable is not declared ahead of time.Such as: console.log(a);var a = 1; is equivalent to a = 1; console.log(a);Advance declaration of functions: functions created using the function declaration form function function(){}It will be created before all the code is executed, so we can call the function before the function declarationFunctions created using function expressions are not declared ahead of time, so they cannot be called before the declaration.

Function scope:

function scope- The function scope is created when the function is called, and after the function is executed, the function scope is destroyed.- Every time a function is called, a new function scope is created, and they are independent of each other.- Variables in the global scope can be accessed in the function scope.Function-scoped variables cannot be accessed in the global scope.- When a variable is manipulated in the function scope, it is first searched in its own scope, and if there is one, it is used directly.If not, look for it in the upper scope.If the global scope is still not found, an error will be reported.For example: var a =10 function(){ var a =1 ; console.log(a);};fun() outputs 1.var a =10 function(){ console.log(a);};fun() The output is 10There is also an early declaration feature in the function scope,Variables declared with the var keyword are declared before all the code in the function is executedIn a function, variables declared without var will become global variables