working with var vs let in for loops

index.js

// the scope of a let variable is the current block scope
// the scope of a var variable is the current function scope (if there is no function, than the scope is global)

// using var in this example will execute the loop and compute var i to the value 10 
// than the timeout callbacks will callback one after the other and access i which is globally set to 10
for(var i = 0; i < 10; i++){
    setTimeout(function () {
        console.log(i);
    }, 1500, i);
}
/*
 * Output:
 * 10
 * 10
 * 10
 * 10
 * 10
 * 10
 * 10
 * 10
 * 10
 * 10
 */
// using let instead means that within every loop scope (block), you are working with another shadowed variable i
for(let i = 0; i < 10; i++){
    setTimeout(function () {
        console.log(i);
    }, 1500, i);
}
/*
 * Output:
 * 0
 * 1
 * 2
 * 3
 * 4
 * 5
 * 6
 * 7
 * 8
 * 9
 */
// even better: use the functional programming paradigm to pass i by value to the inner function of setTimeout(), 
// works as expected for both let and var
for(var i = 0; i < 10; i++){
    setTimeout(function(i){
        console.log(i)
    }, 1500, i);
}
// or even shorter, using an anonymous arrow function: 
for(var i = 0; i < 10; i++){
    setTimeout((i) => console.log(i), 1500, i);
}
/*
 * Output:
 * 0
 * 1
 * 2
 * 3
 * 4
 * 5
 * 6
 * 7
 * 8
 * 9
 */