使用带有 chai-as-promised 的自定义 chai 方法断言

Question

I want to add a custom assertion/method like this:

chai.use(function (chai, utils) {
  var Assertion = chai.Assertion;
  Assertion.addMethod("convertToStringEqual", function (input) {
    new Assertion(this._obj.toString()).to.equal(input.toString());
  });
});

However I want to be able to use it with chai-as-promised like so:

Promise.resolve(2 + 2).should.eventually.convertToStringEqual(4);

But when I run this example, I see this error:

AssertionError: expected '[object Promise]' to equal '4'

This is because chai-as-promised is not resolving that promise with eventually before passing it to convertToStringEqual .

How can I get chai-as-promised await that promise before passing it to my custom assertion method?

Answer 1

Load your custom plugin firstly, then, add the chai-as-promise . Related to the order of loading plugins.

From the #installation-and-setup

Note when using other Chai plugins : Chai as Promised finds all currently-registered asserters and promisifies them, at the time it is installed. Thus, you should install Chai as Promised last , after any other Chai plugins, if you expect their asserters to be promisified.

Eg

const chai = require('chai');
const chaiAsPromised = require('chai-as-promised');

chai.use(function(chai, utils) {
  var Assertion = chai.Assertion;
  Assertion.addMethod('convertToStringEqual', function(input) {
    new Assertion(this._obj.toString()).to.equal(input.toString());
  });
});
chai.use(chaiAsPromised);
chai.should();

describe('65418901', () => {
  it('should pass', () => {
    return Promise.resolve(2 + 2).should.eventually.convertToStringEqual(4);
  });
});

unit test result:

  65418901
    ✓ should pass


  1 passing (52ms)

But, load the plugins like this will not work:

chai.use(chaiAsPromised);
chai.use(function(chai, utils) {
  var Assertion = chai.Assertion;
  Assertion.addMethod('convertToStringEqual', function(input) {
    new Assertion(this._obj.toString()).to.equal(input.toString());
  });
});
chai.should();