Cleaning Up the Tags With a Set

Cleaning Up the Tags With a Set

In this lesson, you cleaned up order 1042’s tags, ["gift", "fragile", "gift"], with a JavaScript Set.

• JavaScript has a Set type too. You make one with the new keyword, as you make a Map, and like Python’s set(), it takes an iterable: new Set(order.tags) builds it from the tags array and drops the repeat.
• For the length, a Set has a .size property, where Python has len(). A Set isn’t an array, so tags.length gives undefined.
• A Set keeps its members in the order they first arrived, so JavaScript logs Set(2) { 'gift', 'fragile' } 2. The number in parentheses is the size, and the braces hold the members: that’s how a Set prints, not code you write.
• Python’s set promises no order. For strings like these, its order can change from run to run: one run printed {'fragile', 'gift'} 2, and the next printed {'gift', 'fragile'} 2.
• Because a Set keeps the order, [...new Set(order.tags)] removes the repeats and gives [ 'gift', 'fragile' ]. The ... spreads the Set into a new array, as it spreads one array into another. In Python, keeping the order takes list(dict.fromkeys(order["tags"])), because a dict keeps its keys in the order they arrived, as a Set does with its members.
• tags.has("gift") does the job of Python’s "gift" in tags, and logs true.
• Watch out for porting in as it reads: JavaScript’s in asks whether an object has a key by that name, as it does for an array’s positions, so "gift" in tags is false.
• .add() adds a member, as in Python, and adding "gift" again leaves the size at 2. Where Python has discard(), a Set has a .delete() method, a call on the Set, not the delete operator you used on object keys.
• Watch out for Python’s set literal: in JavaScript, braces make an object, which needs keys and values, so {"US", "CA", "AU"} ported with its braces stops the file with a syntax error. JavaScript has no Set literal, so pass the items as an array: new Set(["US", "CA", "AU"]).
• Combining sets, such as a union, comes later.

Here’s Marta’s Python step:

order = {"tags": ["gift", "fragile", "gift"]}
country = "GB"
TRACKED_COUNTRIES = {"US", "CA", "AU"}

tags = set(order["tags"])
print(tags, len(tags))
print("gift" in tags)
print(country in TRACKED_COUNTRIES)