karma-json-reporter

// karma.conf.js
module.exports = function(config) {
  // ...
 
    // json reporter directly output stringified json
    reporters: ['json'],
    jsonReporter: {
      stdout: true,
      outputFile: 'results.json' // defaults to none
    }
 
  // ...
};

{
  "browsers": { // Collection of used browser
    "<browser.id>": {
      "id": "<same browser.id>",
      "fullName": String,
      "name": String,
      "state": Number,
      "lastResult": {
        "success":Number,
        "failed": Number,
        "skipped": Number,
        "total": Number,
        "totalTime": Number,
        "netTime":Number,
        "error": Boolean,
        "disconnected": Boolean
      },
      "launchId": Number
    }
  },
  "result": { // Collection result per browser
    "<browser.id>": [
      {
        "id": Number, // spec.id
        "description": String, // spec.description
        "suite": Array.of(String), // spec.suite
        "success": Boolean,
        "skipped": Boolean,
        "time": Number,
        "log": Array.of(String), // spec.log
      },
    ]
  },
  "summary": {
    "success": Number, // total number of success
    "failed": Number, // total number of fail
    "error": Boolean,
    "disconnected": Boolean,
    "exitCode": Number
  }
}