Great stuff. Feature requests: non-wp login, accents, Bar lines, support for non-4/4, swung meter, search by pattern (e.g. write a kick snare pattern, find beats with same kick snare pattern to e.g. find inspiration for hat patterns)
> which might promote misunderstanding of the behavior
This isn't a misunderstanding, binary logical operators in JS short-circuit like this by design. I believe && and || returned a boolean value in the past, but were explicitly changed to support this behaviour.
The production LogicalANDExpression : LogicalANDExpression && BitwiseORExpression is evaluated as follows:
1. Evaluate LogicalANDExpression.
2. Call GetValue(Result(1)).
3. Call ToBoolean(Result(2)).
4. If Result(3) is false, return Result(2).
5. Evaluate BitwiseORExpression.
6. Call GetValue((Result(5)).
7. Return Result(6).
That is, the behavior has always been "If the first expression is false-ish, return the first expression, otherwise return the second expression" ("BitwiseORExpression" is a class of expressions that include a lot of things, including equality operators).
JavaScript does not have any operators that is not explicitly listed in a version of ECMA-262. It would be correct to refer to the construct as a "guard", but incorrect to refer to it as a "guard operator". Calling it "guard operator" also does not promote an understanding of the underlying construct.
ECMA-262 defines ECMAScript. "JavaScript 1.3" refers to a Netscape-specific language implementation ("Mocha", "LiveScript", "JavaScript"), with version 1.3 being based partially based on ECMA-262 Second Edition. Some of it might still live on in Firefox, but it is certainly not what people refer to as "JavaScript".
In general terms, in C-like languages, mixing braces and semi-colons is horribly refactor unfriendly and bug prone. Meanwhile, sticking things on one line introduces a cognitive load that you might not want. (YMMV on the second point, but I’ve pretty strong views on the first.)
