Combine coverage reports from multiple runs in to a single coverage report.

var combine = require('istanbul-combine');

var opts = {
  dir: 'coverage',                       // output directory for combined report(s)
  pattern: 'coverage/*-coverage.json',   // json reports to be combined 
  print: 'summary',                      // print to the console (summary, detail, both, none) 
  base:'sources',                        // base directory for resolving absolute paths, see karma bug
  reporters: {
    html: { /* html reporter options */ },
    cobertura: { /* etc. */ }
  }
};

combine(opts, function(err) { });         // async with node style completion callback
combine(opts).then(/* ... */ );           // async with promise return value
combine.sync(opts);                       // synchronous

npm install -g istanbul-combine

istanbul-combine -d coverage -p summary -r lcov -r html coverage/coverage-a.json coverage/coverage-b.json /coverage/*.json

usage: istanbul-combine [options] patterns

where patterns is any number of file glob-patterns separated by whitespace

options:

• -d : output directory for the report(s). Defaults to coverage
• -p : what to print to the console. summary | detail | both | none. Defaults to summary.
• -r : a reporter. lcov, html, etc. You can specify multiple reporters by using this tag multiple times.
• -b : base directory resolving relative paths to absolute ones. Fixes a bug with where karma reports relative file paths.

You will need to use the base or -b option if you intend to combine reports generated using karma-coverage with reports generated using other tools (i.e. tests not run in the browser). See this pull request.