The documentation for JS_IsArrayObject says /** * Returns true and sets |*isArray| indicating whether |obj| is an Array object * or a wrapper around one, otherwise returns false on failure. * * This method returns true with |*isArray == false| when passed a proxy whose * target is an Array, or when passed a revoked proxy. */ On reading the second sentence, I can interpret the first differently to make it consistent: The function fails whenever |obj| is is not an Array or a wrapper around one; otherwise the function returns true, setting isArray to true if an Array and to false if a wrapper. This is not what the function does, but it took some investigation to settle on that conclusion.

Thank you for looking at my suggestion, Jeff. There are potentially two things involved here: 1. I'm not clear from the first sentence whether *isArray will be set to true or false if |value| is a wrapper around an array. Was the interpretation in comment 0 correct, or alternatively would the proposed change to the first sentence here be accurate? https://phabricator.services.mozilla.com/D6703?id=18237#change-lZV20MqdA9JJ I'm assuming the latter because the function succeeds when !value.isObject(). 2. I don't know enough about the differences between wrappers and proxies. This may be contributing to my confusion. My reading of the code was that ForwardingProxyHandler::getBuiltinClass() would indicate whether the target was an array or not. Is ForwardingProxyHandler a wrapper or a proxy or both? Am I reading the code wrong? https://searchfox.org/mozilla-central/rev/ffe6eaf2f032e58ec3b0650a87df2c62ae4ca441/js/src/proxy/Wrapper.cpp#256

In an attempt to clarify 1, the doc says x is set indicating whether y is A or B. This may be interpreted to mean that, if x is set, then y ∈ {A, B} and the value of x will indicate which from the two possibilities. I assume the intended meaning is instead x is set indicating whether or not y ∈ {A, B}.

Pushed by ktomlinson@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/cd94fba04864 clarify JS_IsArrayObject documentation re proxies r=jwalden