"nyc report" loads all reports in tempDirectory into memory

[RyanV]

Expected Behavior

when a directory has multiple report files, running nyc report ... should read, parse, and merge each report file one by one.

See original istanbul behavior:
https://github.com/gotwarlost/istanbul/blob/master/lib/command/report.js#L109
https://github.com/gotwarlost/istanbul/blob/master/lib/collector.js#L71

Observed Behavior

All reports are read into memory before any merging occurs which, on some machines, causes a FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - Javascript heap out of memory error.

See:
https://github.com/istanbuljs/nyc/blob/master/index.js#L428
https://github.com/istanbuljs/nyc/blob/master/index.js#L517

Background

The project I am working on has many integration tests (1500+) that use instrumented versions of multiple JS builds, each of which write a new coverage report. Each file is between 2.5mb - 4mb.

Temporary Workaround

If anyone else stumbles upon this and needs a temporary workaround, I wrote a quick basic script to merge the results to a single file which you can then use when calling nyc report ...
https://gist.github.com/RyanV/6fe44b80082a8eb609199bfb0e8da953

Or if you have the memory to spare, increase the heap size allocation by passing the flag node --max-old-space-size=[size] nyc ..., where [size] is a number value in megabytes.

Forensic Information

OS: Mac, Windows
Node Version: 7.9.0
NPM Version: 4.2.0