Reading Characters With [] and at()

Reading Characters With [] and at()

In this lesson, you ported the helper’s full-stop check and read single characters the JavaScript way. The review bot rejects a commit subject that ends with a full stop, and PAY-2304’s title, Retry failed card payments., has one.

• Python’s len() function becomes the length property in JavaScript, read after a dot with no parentheses: title.length is 27. Watch out: len() doesn’t exist in JavaScript, and title.length() is a TypeError, because a property isn’t a function.
• Brackets read a position counted from the start, as in Python: title[0] is R, and title[26], one less than the length, is the full stop.
• JavaScript’s brackets don’t count from the end: they look up a position by its number, and the string has no position -1. The natural port of the check, const last = title[-1];, gives undefined, JavaScript’s value for nothing there, so the check prints Ends with a full stop: false where Python prints True, with no error.
• Reading a position that isn’t there gives undefined in JavaScript, with no error. A position past the end gives undefined too, where Python raises an IndexError, so check the length when the text might be short.
• To count from the end, use the at() method: a negative position counts back from the end, as in a Python index, so title.at(-1) is the full stop. Like the brackets, at() gives undefined, not an error, when the position isn’t there.
• at() takes positions from the start too, but JavaScript code keeps the brackets for those and reaches for at() when it counts from the end.
• As in Python, a string can’t change in place. In the helper’s port, title[0] = "r" stops with a TypeError.
• For text like these titles, each position holds one character. Text with emoji is where JavaScript’s positions and Python’s part ways, a later topic.

Here’s the check with the fix in place:

const title = "Retry failed card payments.";

const last = title.at(-1);
console.log(`Ends with a full stop: ${last === "."}`);