Doubts about the following code

Basically it's the different scope of var with respect to let , here You can find documentation of it in Spanish.

Now, here is the explanation, as mentioned in the documentation, the scope of var is greater than let , so for example in the code when you declare in the% var i = 0 the reach is global , the variable is "raised" to the window object in the case of the browser. This behavior does not occur with let , since let i = 0 is not "raised", but belongs to the for, its scope is only inside the for.

for (var i_var = 0; i_var < 10; i_var++) {
    setTimeout(function() { console.log(i_var); }, 100 * i_var);
}
console.log("Valor de var: "+i_var);
for (let i_let = 0; i_let < 10 ; i_let++) {
    setTimeout(function() { console.log(i_let); }, 100 * i_let);
}
console.log("Valor de let: "+i_let);

When executing that code, the error comes out that i_let is not defined, because it was not raised to window, once this is clarified now the answer does come:

At the end of the first for is shown the value of i_var , that is 10 and each console.log() of setTimeout will refer to that value, because as you will have noticed, first print "Value of var: 10 ", then the error and then the others console.log() are executed.

And in the second for it is different, because if the for and the variable i_let no longer exists, what is happening? simply every console.log() of the second for is referring to a different variable, each iteration represents a different value.

Therefore, even though the console.log() is executed when the for and i_let finished, it no longer exists, you can print a value, this is the variable that was created in the cycle and its value corresponds to the number of iteration.

The reserved word var , creates a variable which can be used by its own scope (reach) and functions INSIDE of its scope, instead the word let , it is limited only to its own scope.

Example of var :

(function() {
var f = "Hola";
(function(){
console.log(f); // Muestra correctamente
})(); 
})();

Example of let :

(function() {
    let f = "Hola";
    (function(){
    console.log(f); // NO muestra correctamente
    })(); 
    })();