Canonicalize objects before stringifying and diffing them

[papandreou]

Assertion libraries can set the showDiff of the thrown Error instances to true, which makes mocha serialize the actual and expected properties, then do a string diff -- effectively a poor man's object diff. However, it produces an unnecessarily big diff when the key definition order in objects are different:

var expect = require('unexpected');

describe('the thing', function () {
    it('should DTRT', function () {
        expect({foo: 123, bar: 456, quux: 789}, 'to equal', {quux: 78, foo: 123, bar: 456});
    });
});

  0 passing (5ms)
  1 failing

  1) the thing should DTRT:

      + expected - actual

       {
      +  "quux": 78,
         "foo": 123,
      +  "bar": 456
      -  "bar": 456,
      -  "quux": 789
       }

This patch canonicalizes objects before comparing them so the above example produces:

  0 passing (5ms)
  1 failing

  1) the thing should DTRT:

      + expected - actual

       {
         "bar": 456,
         "foo": 123,
      +  "quux": 78
      -  "quux": 789
       }

... which I've found to be a great help when diffing large objects.

[rprieto]

This would be great. I was about to submit a similar patch using json-stable-stringify, which should be a single-line fix:

base.js

var stableStringify = require('json-stable-stringify');

// line 456
// return JSON.stringify(obj, null, 2);
   return stableStringify(obj, {space: 2, cycles: true})

If we don't mind adding a dependency, let me know if we're interested in it:
https://github.com/rprieto/mocha/commit/dc324f6a063569da691dffcb992f603bb30b9f1f

[travisjeffery]

makes sense. could you write a test though? thanks

[ai]

Good idea. I have often problems with different order props in objects.

[papandreou]

@travisjeffery All right. Should the test be added to https://github.com/visionmedia/mocha/blob/master/test/acceptance/diffs.js? I have a little trouble understanding how the tests are structured.

I'm on holiday for the rest of the year, so if someone with time on their hands wants to see this land sooner, don't hesitate to take over :)

[travisjeffery]

💥 done