What is scope in javascript
In JavaScript, scope is the accessible range of variables (objects, functions), and the effective range of variables that can be read and written in script code; scope can control the visibility and life cycle of variables. .
The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.
Almost all languages have the concept of scope. Simply put, scope (scope) is the accessible scope of a variable, that is, the scope controls the visibility and life cycle of the variable. .
In JavaScript, objects and functions are also variables.
Before ECMAScript6, the scope of variables was mainly divided into global scope and local scope (also called function scope); in ECMAScript6 and after, the scope of variables was mainly divided into There are three types: global scope, local scope and block-level scope.
The variables in the corresponding scope are called global variables, local variables and block-level variables respectively.
Global variables are declared outside all functions;
Local variables are variables declared within the function body or named parameters of the function;
Block-level variables are variables declared in the block and are only valid in the block.
The scope of a variable is closely related to the declaration method. Variables declared using var have global scope and function scope, and there is no block-level scope; variables declared using let and const have global scope, local scope and block-level scope.
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The above script code declares 4 global variables, 3 local variables and 1 block-level variable respectively. Outside the scopeTest function, variables v1, v2, v3 and v4 are global variables; within the scopeTest function body, lv and v2 are global variables; in the if judgment block, lv is a block-level variable.
We see that local variables v1 and v2 have the same names as global variables v1 and v2. In the scopeTest function body, local variables v1 and v2 are valid, so the output results of these two variables in the function body are "bbb" " and "ccc"; outside the function body, the global variables v1 and v2 are valid, so outside the function body, the output results of these two variables are "JavaScript" and "JScript" respectively.
In addition, the block-level variable lv and the local variable lv have the same name. In the if judgment block, the block-level variable lv is valid, so the output result in the block is "123", while outside the block, the local variable lv Valid, the output result of the lv variable is "aaa".
In addition, the global variables v3 and v4 are not overwritten in the function body, so the value of the global variable is output, so the output result of v3 in the function body and outside the body is "Script", and the v4 variable The assignment is after the function call, so the output result of v4 in the function body is "undefined", while the output outside the function body is after the declaration, so the result is "VBScript". lv is a local variable, so accessing it outside the function will report a "ReferenceError" error.
After the above code is run in the Chrome browser, open the browser's console and you can see the output shown in the figure below.
The reason for the error in line 28:
The lv variable is a local variable and is invalid after leaving the function. Comment this line of code and then run it. At this time, open the browser console and you will see:
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