Using IsCallable Operation?

Mark S. Miller erights at
Fri Jun 5 14:18:58 PDT 2009

On Fri, Jun 5, 2009 at 1:39 PM, Allen Wirfs-Brock
<Allen.Wirfs-Brock at> wrote:
> Not in ES5.  Generally checking that typeof x === 'function' is a close approximation. ES5 (11.4.3) requires that  both native (ie, pure actual ECMAScript objects) and host objects that implement [[Call]] produce "function" when typeof is applied to them.  This is a change from ES3 that did not impose this requirement upon host object objects.

Why is this an approximation? In ES5, how can
    typeof x === 'function'
differ from testing whether something is callable?


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