Proper noun
E4X
(computing) Initialism of ECMAScript for XML (the ECMA-357 standard)
First, Brendan Eich and Mozilla rejoined Ecma International as a not-for-profit member and work started on ECMAScript for XML (E4X), the ECMA-357 standard, which came from ex-Microsoft employees at BEA Systems (originally acquired as Crossgain). Source: Internet
E4X objects can no longer be inspected in 1.4.0 (new XML rep type). Source: Internet