Use Custom Message in Chai

When running a test in a loop with Mocha and Chai, it might not be easy to pinpoint the element where the error occurred. For example:

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for (let i = 0, len = arr.length; i < len; i += 1) {
  expect(arr[i]).to.equal(true);
}

Run the test:

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$ mocha --harmony test/message.js
  Custom Message
    1) Should return true
  0 passing (37ms)
  1 failing
  1) Custom Message Should return true:
      AssertionError: expected false to equal true
      + expected - actual
      -false
      +true
      at Context.<anonymous> (test/message.js:16:25)

It failed on the expect statement, but the error message does not show which element is in error.

Instead, we can add a custom message to the expect statement:

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for (let i = 0, len = arr.length; i < len; i += 1) {
  expect(arr[i], 'i = ' + i).to.equal(true);
}

Now, when running the test, you can pinpoint the source of the error:

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$ mocha --harmony test/message.js
  Custom Message
    1) Should return true
  0 passing (17ms)
  1 failing
  1) Custom Message Should return true:
      i = 1
      + expected - actual
      -false
      +true
      at Context.<anonymous> (test/message.js:16:37)

The error occurs at the second element.

Reference:

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// Custom Message in Chai
// ======================
'use strict';
describe('Custom Message', function () {
  // Dependencies
  // ------------
  var expect = require('chai').expect;
  var arr    = [ true, false, true ];
  it('Should return true', function () {
    for (let i = 0, len = arr.length; i < len; i += 1) {
      expect(arr[i], 'i = ' + i).to.equal(true);
    }
  });
});