Scope.js

function loop(arr) {
    // i IS NOT known here
    // j IS NOT known here

    for( var i = 0; i < arr.length; i++ ) {
        // i IS known here
    }

    // i IS known here
    // j IS NOT known here

    for( let j = 0; j < arr.length; j++ ) {
        // j IS known here
    }

    // i IS known here
    // j IS NOT known here
}

///Here, we can see that our variable j is only known in the first for loop, but not before and after.
//Yet, our variable i is known in the entire function from the moment it is defined onward.