Are trailing commas in arrays and objects part of the spec?
Just a quick reminder/warning that this is one of the areas in which the JavaScript/ECMAScript standard and JSON standard differ; trailing commas are valid in JS but not valid in JSON.
Specs: ECMAScript 5 and ECMAScript 3
Section 11.1.5 in the ECMAScript 5 specification:
ObjectLiteral :
{ }
{ PropertyNameAndValueList }
{ PropertyNameAndValueList , }
So yes, it is part of the specification.
Update: Apparently this is new in ES5. In ES3 (page 41), the definition was just:
ObjectLiteral :
{ }
{ PropertyNameAndValueList }
For arrays literals (Section 11.1.4) it is even more interesting (Update: this already existed in ES3):
ArrayLiteral :
[ Elisionopt ]
[ ElementList ]
[ ElementList , Elision_opt ]
(where Elision_opt
is Elisionopt, meaning the Elision is optional)
Elision
is defined as
Elision :
,
Elision ,
So, an array literal like
var arr = [1,2,,,,];
is perfectly legal. This creates an array with two elements but sets the array length to 2 + 3 = 5
.
Don't expect too much from IE (before IE9)...