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Welcome to JavaScript! You've stumbled onto something every beginner hits eventually: reserved words.
class is a keyword that JavaScript itself uses for defining classes. Because the language reserves it for its own use, you can't use it as a variable name. That's why your code breaks the moment the parser sees let class = ...
This is also why your experiments behaved the way they did:
Class (capital C) worked because JavaScript is case-sensitive, and only the lowercase class is reserved.
myclass worked because it's just a regular identifier that happens to contain the word.
"class" in quotes became a string literal, which is valid syntax but meant something completely different (hence the new error).
The fix
Just rename the variable to something that isn't reserved:
let teacher = "Mr. Henderson"; let room = 204; let subject = "Biology"; // <-- renamed from `class` let period = 3; console.log(teacher + " teaches " + subject + " in room " + room);Other reserved words to avoid
Some common ones that trip beginners up: class, function, return, if, else, for, while, let, const, var, new, this, super, import, export, default, delete, typeof, instanceof, null, true, false.
The full list is on MDN's reserved words page if you're curious.
