for-in blocks in array comprehensions are a Mozilla-specific language extensions that is no longer supported even by SpiderMonkey, and is unlikely to be included in future ECMAScript standards. This language feature should not be used.

The for-in block can be replaced by a (standards-compliant) for-of block iterating over a list of property names obtained, for example, from Object.keys.

In the following contrived example, a for-in block is used to iterate over the keys i of an array and construct an array of strings of the form "v = a[i]", where v is the value of a[i].

The example can be rewritten to use a for-of block iterating over Object.keys(a) instead.

Note that Object.keys only includes own properties, not properties inherited from a prototype. If the latter behavior is needed, the array comprehension should be replaced by a for-in loop that imperatively populates the result array.

Mozilla Developer Network: Array comprehensions: Differences to the older JS1.7.2/JS1.8 comprehensions.