Javascript - What does (1, 2) actually mean?

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

The individual expressions - the bits between the commas - are evaluated, then the overall expression takes the value of the last one. So listing a series of numeric and string literals like your example is kind of pointless:

(1, 'a', 5, ... 'b')
// does the same thing as
('b')

...so you might as well leave out all but the last. However if the individual expressions have other effects because they are function calls or assignments then you can't leave them out.

About the only good reason I can think of to use this syntax is within a for statement, because for syntax:

for([init]; [condition]; [final-expression])

...doesn't allow you to use semicolons within the [init] part, or within the [condition] or [final-expression] parts. But you can include multiple expressions using commas:

for(x = 0, y = 100, z = 1000; x < y; x++, y--, z-=100) { ... }