A small problem with js

related codes

var a = "aa";
test ();
function test () {
console.log (a);
var a = "bb";
console.log (a);
}
console.log (a);
/ / undefined bb aa

I would like to ask why the undefined is printed in the first place. Isn"t it possible to access variables outside the function inside the function

Javascript
Jun.30,2021

because you also redefine a below, an is considered to be a local variable , so when test () runs to the first console.log (a) , the value of an is undefined , and then it will be assigned to bb .

to understand the execution order of var a = 'bb' , execute var a = undefined first, and then assign bb to a until the assignment statement is executed.

to pass an as an argument to the function

you can take a look at the problem of variable promotion. If the variable is redefined within the function, the variable will also be promoted in the local environment, with a value of undefined

.

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