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/* What will the following code output? */
for (var i = 0; i < 3; i++) {
  setTimeout(function() { alert(i); }, 1000 + i);
}

/* ANSWER */
/*
it will show - 3, 3, 3. 
But we expect: 0, 1, 2.
one way to solve the problem is immediately executable function
for (var i = 0; i < 3; i++) {
  setTimeout(function(i_local) { 
    return function() { alert(i_local); } 
  }(i), 1000 + i);
}

или лучше просто использовать let вместо var.
for (let i = 0; i < 3; i++) {
  setTimeout(function() { alert(i); }, 1000 + i);
}
*/