gulpfile.js

var gulp = require('gulp'),
  mocha = require('gulp-mocha'),
  istanbul = require('gulp-istanbul');

var src_path = 'src/*.js';
var test_path = 'test/*.js';

gulp.task('pre-test', function () {
  return gulp.src([src_path])
    // Covering files
    .pipe(istanbul())
    // Force `require` to return covered files
    .pipe(istanbul.hookRequire());
});

gulp.task('test', ['pre-test'], function () {
  return gulp.src([test_path])
    .pipe(mocha())
    // Creating the reports after tests ran
    .pipe(istanbul.writeReports())
    // Enforce a coverage of at least 90%
    .pipe(istanbul.enforceThresholds({ thresholds: { global: 90 } }))
    ;
});

package.json

{
  "name": "testing-test",
  "version": "1.0.0",
  "description": "Testing istanbul",
  "main": "src/index.js",
  "scripts": {
    "test": "gulp test"
  },
  "author": "Daniel Fath <daniel.fath7@gmail.com>",
  "license": "ISC",
  "devDependencies": {
    "gulp": "^3.9.1",
    "gulp-istanbul": "^1.0.0",
    "gulp-mocha": "^2.2.0"
  },
  "dependencies": {
    "chai": "^3.5.0"
  }
}

src\index.js

(function(exports) {
  'use strict';


  exports.testing = function() {
    if (arguments[0] !== null) {
      return arguments[0];
    } else {
      return -1;
    }
  };

}(typeof exports === 'object' && exports || this));

test\index.js

var chai = require('chai');
var lib = require('../src/index.js').testing;
var should = chai.should();

describe('#test', function() {
  it('test', function(){
    lib("x");
  });
});