agecheck-commented.js

// using for-loop structure to perform an action until a condition is satisfied
for(
  // for-loop initialization:
  // create a var which is a function that returns 0, 1 or 2, given an age
  check = age =>
    // check for underage
    age < 18
      ? 0
      // check for oldness. what would a golfed loop be without a nested ternary?
      : age < 66
        ? 1
        : 2,
  
  // also create a var for the age check result
  result = 0;
  
  // for-loop condition:
  // 0 or 1 keep the loop running, 2 to stop
  result < 2;
  
  // for-loop action:
  result = check(
    // using template literal syntax to call a function with 2 less chars
    prompt`ikä`
  ), // using a comma to separate 2 expressions within a for-loop statement, as semicolons cannot be used here
  alert(
    // result doubles as an index for the alert messages
    ['lapsi','aikuinen','vanhus'][result]
  )
) {
  // the empty for-loop block
}

agecheck.js

for(c=a=>a<18?0:a<66?1:2,r=0;r<2;r=c(prompt`ikä`),alert(['lapsi','aikuinen','vanhus'][r])){}