How to use the gulp-istanbul.writeReports function in gulp-istanbul

coveragePost() {
        chai.should();
        // chai.use(require("sinon-chai"));
        // chai.use(require("chai-as-promised"));

        return gulp.src([
            "./build/compiled/test/functional/**/*.js",
            "./build/compiled/test/issues/**/*.js",
        ])
            .pipe(mocha())
            .pipe(istanbul.writeReports());
    }

function runner() {
    return gulp
      .src(testGlob)
      .pipe(mocha(mochaOpts))
      //      .on('error', function (err) {
      //        gutil.log(err);
      //        this.emit('end');
      //      })
      .pipe(istanbul.writeReports())
      .on('end', cb);
  }

coveragePost() {
        chai.should();
        chai.use(require("sinon-chai"));
        chai.use(require("chai-as-promised"));

        return gulp.src(["./build/compiled/test/**/*.js"])
            .pipe(mocha())
            .pipe(istanbul.writeReports());
    }

function test() {
    gulp.src(['./test/**/*.js'], { read: false })
        .pipe(mocha())
        .pipe(istanbul.writeReports({
            dir: 'test-coverage/',
            reportOpts: {
                dir: 'test-coverage/'
            },
            reporters: ['lcov', 'text', 'text-summary', 'cobertura']
        }))
        .pipe(istanbul.enforceThresholds({ thresholds: { global: 1 } }));
    return gulp.src('test-coverage/lcov.info')
        .pipe(coveralls());
}

function test() {
    gulp.src(['./test/**/*.js'], { read: false }).pipe(mocha()).pipe(istanbul.writeReports({
        dir: 'test-coverage/',
        reportOpts: {
            dir: 'test-coverage/'
        },
        reporters: ['lcov', 'text', 'text-summary', 'cobertura']
    })).pipe(istanbul.enforceThresholds({ thresholds: { global: 1 } }));
    return gulp.src('test-coverage/lcov.info').pipe(coveralls());
}

gulp.task('test', ['pre-test'], function (cb) {
    var mochaErr;

    gulp.src('test/**/*.js')
        .pipe(plumber())
        .pipe(mocha({reporter: 'spec'}))
        .on('error', function (err) {
            mochaErr = err;
        })
        .pipe(istanbul.writeReports())
        .on('end', function () {
            cb(mochaErr);
        });
});

.on('finish', function(){
      gulp.src(paths.tests)
        .pipe(mocha())
        .pipe(istanbul.writeReports());
    });
});

.on('finish', function () {
            gulp.src(files.test)
                .pipe(mocha({reporter: 'dot'}))
                .pipe(istanbul.writeReports({
                    dir: './coverage',
                    reporters: ['lcov', 'json', 'html'],
                    reportOpts: { dir: './coverage' }
                }))
                .on('end', done);
        });
});

function runner() {
    gulp
      .src(['./test/main.js'])
      .pipe(mocha(mochaOpts))
      .pipe(istanbul.writeReports())

    .on('end', cb);
  }

.on('finish', function() {
      gulp.src(paths.testFiles)
        .pipe(mochaDefault)
        .pipe(istanbul.writeReports(paths.coverageDir));
    });
});