Counting With the Classic for Loop

Counting With the Classic for Loop

In this lesson, you ported the night-bus report’s timetable loop from Python to JavaScript.

• JavaScript has no range(). You count with the three-part for loop. Python’s range(10, 60, 20) becomes a start (let minute = 10), a condition (minute < 60) and a step (minute += 20), separated by semicolons inside the header’s parentheses.
• The header runs in order: the start runs once, then the condition is checked, the body runs, the step runs, and the condition is checked again, until it’s false. In Python, that’s a while loop with the step as the body’s last line, though range() stays the way to write it there.
• The rule from for...of still holds: const unless something reassigns the name. In a for...of loop, nothing does, because each pass gets a fresh variable. In a counting loop, the step reassigns the counter on every pass, so the counter is a let. A const counter stops the loop with a TypeError as soon as the first step runs.
• The counter exists only inside the loop. Python’s loop variable keeps its last value after the loop, but in JavaScript, reading the counter after the closing brace is a ReferenceError.
• To count down, start high, flip the condition, and subtract in the step. range() reads the direction from the step’s sign, but the condition is a plain test, so it has to point the way you count.
• To count by one, the step is i++, which adds one to i, like i += 1.
• For an array’s items rather than numbers, for...of is still the loop to write.

The finished timetable loop:

for (let minute = 10; minute < 60; minute += 20) {
  console.log(`N26 leaves at 23:${minute}`);
}