for (var i = 0; i < 4; i++) {
// global scope
setTimeout(() => console.log(i));
}
for (let i = 0; i < 4; i++) {
// block scope
setTimeout(() => console.log(i));
}The output of the above for loops is 4 4 4 4 and 0 1 2 3
Explanation: Due to the event queue/loop of javascript, the setTimeout callback function is called after the loop has been executed. Since the variable i is declared with the var keyword it became a global variable and the value was equal to 4 using iteration when the time setTimeout function is invoked. Hence, the output of the first loop is 4 4 4 4.
Whereas in the second loop, the variable i is declared as the let keyword it becomes a block scoped variable and it holds a new value(0, 1 ,2 3) for each iteration. Hence, the output of the first loop is 0 1 2 3.
