The boolean metadata is a type hint telling the ClojureScript compiler that
is-hidden is boolean. Knowing is-hidden is a boolean allows ClojureScript to
compile to
if (is-hidden){
}
else {
}
instead of
if (is-hidden != null && is-hidden !== false){
}
else {
}
if the type isn't known to be a boolean. The extra checks are needed because
ClojureScript has only two values that evaluate to false: false and nil.
JavaScript evaluates false, null undefined, "", 0, and NaN to be false.
About the only time you need to use the boolean type hint is in performance
sensitive code where the compiler can't infer the type of a var (like a
parameter passed into a function).
I hope this answers your question
Peter Schuck
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