Trailing Commas in Arrays and Objects: Standard or Tolerated?

The presence of trailing commas in arrays and objects has sparked some debate regarding their standardization in JavaScript. This question stems from the inconsistent behavior observed in different browsers, particularly older versions of Internet Explorer.

Specification Status

According to the ECMAScript 5 specification (Section 11.1.5), trailing commas are explicitly allowed in object literals:

ObjectLiteral :
    { }
    { PropertyNameAndValueList }
    { PropertyNameAndValueList , }

Therefore, trailing commas in object literals are part of the official JavaScript specification.

ES3 vs. ES5

Interestingly, trailing commas in object literals were not part of the ECMAScript 3 specification. However, they were introduced in ECMAScript 5 as an optional feature.

Arrays and Trailing Commas

The ECMAScript 5 specification (Section 11.1.4) also allows trailing commas in array literals:

ArrayLiteral :
    [ Elisionopt ]
    [ ElementList ]
    [ ElementList , Elision_opt ]

"Elision" refers to a comma with no associated element, which means arrays can contain empty elements. Therefore, an array like [1,2,,,,] is valid JavaScript, creating an array with two elements but a length of five.

Browser Compatibility

While trailing commas are part of the JavaScript specification, browser support has varied over time. Older browsers like IE8 may have encountered issues with trailing commas, but modern browsers like Chrome and Firefox fully support them.

In conclusion, trailing commas in arrays and objects are part of the ECMAScript specification, both in ES5 and later versions. This feature provides flexibility in code readability and maintainability and is widely supported by modern browsers.