Question

I know that in javascript the syntax (1, 'a', 5, ... 'b') will always return the last value but what does this syntax actually mean? When I see (1, 2) — which, admittedly is pretty much never — how should I parse that syntax?

Answer 1

That’s just the comma operator, which evaluates two expression and returns the second one. Since it evaluates arguments from left to right, if you have a list of arguments separated by commas the last one will be returned. From The Elements of JavaScript Style:

The comma operator was borrowed, like much of JavaScript’s syntax, from C. The comma operator takes two values and returns the second one. Its presence in the language definition tends to mask certain coding errors, so compilers tend to be blind to some mistakes. It is best to avoid the comma operator, and use the semicolon statement separator instead.