The formal definition of the core JavaScript language is embodied in the ECMA standard 262.

Until recently, where implementations conformed it was to the second edition of the standard, which was published in August 1998.

In December 1999, the third edition was made available and new implementations or revisions to existing products released in the early part of the year 2000 onwards complied with that new version of the standard.

To comply with the standard, an implementation must provide and support all the types, values, objects, properties, methods, and program syntax described in the standard.