In for loop different output for let and var?

Question

in first loop am getting print of console 3 of 3 times in second loop am printing console 0,1,2

/*  this loop am getting print console of 3 for 3 times */

for (let i = 0; i < 3; i++) {
  setTimeout(function log() {
    console.log(i);
  }, 1000);
}


/*  this loop am getting print console of 0, 1, 2 */

for (var i = 0; i < 3; i++) {

  setTimeout(function log() {
    console.log(i); // What is logged?
  }, 1000);
}