JS variables and their scopes, JS variable scopes

JS variables and their scopes, JS variable scopes

1. variables and their scopes: variables are classified into "global variables" and "local variables". The "global variables" are declared outside the function and can be used by all functions, the "local variable" is declared within the function body and can only be used in the function that defines the variable.

(Note: When declaring a variable, there is no var keyword, but the variables directly assigned values are global variables)

<Script type = "text/javascript"> function main () {n = 10; // here, n is a global variable and can be directly used externally} main (); alert (n); </script>

2. In the function body, the priority of local variables is higher than that of global variables.

<Script type = "text/javascript"> // an example I think is very representative on the Internet. variables with the same name are declared both inside and outside the function body, for example, var n = 1; function test () {alert (n); // here, a is not a global variable, the reason is that the fourth row of the function declares a local variable with the same duplicate name // a. If the Declaration of the Fourth Row a is commented out, the expression a here shows 1, which is a global variable. So I have to // come to the conclusion that global variable a is overwritten by local variable. // It indicates that before the JS function test () is executed, variable a in the function directs to a local variable, but the output in this // row is not assigned a value during execution, so undefined is displayed. N = 2; alert (n); var n; // returns the local variable a alert (n) ;}test (); alert (n); </script>

According to my understanding, the final output answer of the above example should be: 1 2 2 1; but the correct answer is: undefined 2 2 2 1; this is because local variables overwrite global variables when variables with the same name are declared both inside and outside the function body.

3. How can I read the local variables inside the function body from the outside?

Generally, only the function body can directly obtain external global variables, but it cannot obtain local variables inside the function body. However, by defining a function inside the function body to return a local variable, you can call the function externally.

<Script type = "text/javascript"> function f1 () {var n = 10; function f2 () {// define f2 () inside f1 (), access the local variable alert (n);} return f2; // return the f1 () local variable n} var result = f1 (); // call the f1 () function externally to obtain the result () of the local variable n; // 10, that is, the value of n </script>

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