You should use it only if you want run tests in IE8 and lower, otherwise consider karma-chai-plugins, which provides more complete Chai assertions as well as additional plugins.

Install the module from npm:

$ npm install karma-expect --save-dev

Add expect to the frameworks key in your Karma configuration:

module.exports = function(karma) {
  karma.set({

    // frameworks to use
    frameworks: ['mocha', 'expect']

    // ...
  });
};

Expect.js assertions are available in the tests:

describe('karma tests with expect', function() {
  it('should expose expect method', function() {
    expect('foo').to.not.equal('bar');
  });
});