letVsVar.js

// prints '5' 5 times
// yikes that's not what we want
// the code runs so fast and creates only one binding of i equal to its last value, 5
for (var i = 0; i < 5; i++) {
    setTimeout(function () {
        console.log(i);
    }, 1000);  
}

// solution 1: pass setTimeout a callback with arguments
// caveat: only modern browsers support the third argument
// prints 0, 1, 2, 3, 4
for (var i = 0; i < 5; i++) {
  setTimeout(print, 1000, i);
}

function print(i) {
  console.log(i);
}

// solution 2: wrap SetTimeout in a closure
// works in all browsers
// prints 0, 1, 2, 3, 4
for (var i = 0; i < 5; i++) {
  handleTimeout(i);
}

function handleTimeout(i) {
  setTimeout(function () {
    console.log(i);
  }, 1000); 
}

// solution 3: ES6 to the rescue! just use 'let'
// prints 0, 1, 2, 3, 4
for (let i = 0; i < 5; i++) {
    setTimeout(function () {
        console.log(i);
    }, 1000);  
}