Countdown

Writing a line once and running it many times

A countdown is one sentence said over and over with a different number in it. You could write five separate console.log lines by hand, but five is the gentle case: a program greeting a thousand people cannot be typed out.

A for loop is how you write the line once and say how many times to run it.

for (let n = 1; n <= 4; n++) {
  // the lines in here run four times
}

Everything the loop needs sits between the round brackets, in three parts separated by semicolons. Read them as three questions.

• Where does it start? let n = 1 makes a counter called n and puts 1 in it, once, before the first pass.
• How long does it keep going? n <= 4 is checked before every pass. True and the block runs again; false and the loop is over and JavaScript carries on below it.
• What changes each time? n++ happens at the end of every pass. Two plus signs mean add one; two minus signs mean take one away.

After the brackets come curly braces, doing the job they did for if: the lines inside run once per pass, and the lines after the closing brace run once, when the counting is done.

Why this one cannot be const

Every name you have made so far has been a const, which keeps its value for good.

A counter is the opposite: its entire purpose is to hold a different number on every pass. Write for (const n = 1; n <= 4; n++) and the block runs once, then the program stops with TypeError: Assignment to constant variable. — n++ was told to change something that had been fixed.

let is the other way to name a value, and this is what it is for: a name meant to move. Reach for const everywhere else, so let stays a signal that this one changes.

A worked example

Seats rather than a rocket, so the countdown stays yours to write:

for (let seat = 1; seat <= 3; seat++) {
  console.log("Seat", seat, "ready");
}
console.log("All seats checked.");

Seat 1 ready
Seat 2 ready
Seat 3 ready
All seats checked.

Three passes, and seat holds a different number on each one without you ever setting it again. The last line sits outside the braces, so it runs once, after the loop has finished.

Your turn

The editor holds a launch sequence that already runs. Press Run before changing anything and the trouble shows itself: the count reads 0, 1, 2 and then announces liftoff. No rocket has ever left the ground that way.

One line needs rewriting, the one beginning with for:

for (let i = 0; i <= 2; i++) {

The banners, the rocket and the printing line between the braces are finished — leave them alone.

Rewrite the three parts so the count reads 5, 4, 3, 2, 1 and then hands over to the liftoff banner. Start at five, keep going while the counter has not yet dropped below one, and take one away each pass instead of adding one.

If something goes wrong

A countdown one number too long or too short is the likeliest result, and it is usually the middle part. The test is asked before each pass, so the last number printed is the last one for which it was still true. Let the test survive zero and a T minus 0 line sneaks in before the banner. If the count starts on the wrong number instead, it is the first part rather than the test.

The next likeliest is a loop that never finishes, which happens when the counter travels away from the finish rather than towards it. The page notices, stops the run and says so. Nothing is damaged; the step simply needs to move the counter the way the test is waiting for.

And if nothing appears between the two banners, the loop took no passes: the test was already false on the very first check. Read the three parts left to right as start, keep going, change, then run it again.