Trailing Commas in Arrays and Objects: Are They Standard in JavaScript?

Trailing Commas in Arrays and Objects: Are They Standard?

In the realm of JavaScript, the question arises: Are trailing commas in arrays and objects part of the ECMAScript specification?

When it comes to arrays, Section 11.1.4 of the ECMAScript 5 specification states that an ArrayLiteral can include an "Elision_opt", which is essentially an optional trailing comma. This means that arrays like var arr = [1,2,,,,]; are perfectly legal, creating an array with two elements and an array length of 5.

In contrast, arrays in ES3 (page 41) did not allow for trailing commas in their definition.

For objects, Section 11.1.5 of the ECMAScript 5 specification explicitly allows trailing commas in ObjectLiterals. However, this feature was introduced in ES5; in ES3, trailing commas were not part of the definition.

Despite the specification's inclusion of trailing commas, Internet Explorer (prior to IE9) may not support this syntax. Therefore, while trailing commas are indeed part of the ECMAScript standard, it is advisable to exercise caution when using them in environments that might encounter older browser versions.

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