Testing with Mocha: Array Comparison

The problem

I was writing a mocha test for an array comparison. Here is the test suite:

describe(‘Array comparison’, function () { ‘use strict’; it(‘should return true if two arrays have the same values’, function () { var myArray = [ ‘a’, ‘b’, ‘c’ ]; expect(myArray).to.equal([ ‘a’, ‘b’, ‘c’ ]); }); });

However, out of my expectation, this test would fail:

AssertionError: expected [ ‘a’, ‘b’, ‘c’ ] to equal [ ‘a’, ‘b’, ‘c’ ]

My Explanation

Why do arrays not compare the way other values do? Because typeof array is an object. In mocha, to.equal does not signify that the operands are semantically equal, but they refer to the exact same object. In other words, the above test fails since myArray is not the exact same object as [ ‘a’, ‘b’, ‘c’ ];

Possible Solutions

  1. Use .eql for ‘lose equality’ in order to deeply compare values.
  2. Use .deep.equal, which does not test whether the operands are the same object, but rather that they are equivalent.
  3. Check .members in the array instead
  4. Convert the array to string and compare it.


http://chaijs.com/api/bdd/#arguments-section http://chaijs.com/api/bdd/#members-section

Originally published at victorleungtw.com on November 28, 2014.


Years of experience delivering enterprise projects with global stakeholders in banking, airline and gaming industries involving a broad range of technologies. Provide consulting services at both executive and operational levels, including strategy and data analysis. Lead web and mobile app development team with hands-on coding implementation. Manage projects as Certified Scrum Master with Agile techniques. Qualification with master degree in Computer Science and pursuing MBA degree in Finance.

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