Odd Behavior: No Stack Trace and Double Output

[skeggse]

I'm not sure how to go about debugging this scenario. I've narrowed it down to the following snippet. It's plausible that chai is causing this, but I don't know enough about the guts of either chai or mocha to figure out which it is.

Node v0.10.26, Mocha 1.18.2, Chai 1.9.1

Given

var chai = require('chai');
chai.config.includeStack = true;
var expect = chai.expect;

describe('Woops', function() {
  it('should not fail', function() {
    var a = {}, b = {};
    a.b = b;
    b.a = a;
    expect(a).to.eql({});
  });
});

Output

Given

This next bit is probably just an extension of the above issue, but it exhibits slightly weirder behavior.

var chai = require('chai');
chai.config.includeStack = true;
var expect = chai.expect;

describe('Woops', function() {
  it('should not fail', function() {
    var a = {}, b = {};
    a.b = b;
    b.a = a;
    expect(a).to.eql({});
  });

  it('should succeed', function() {});
});

Output

[skozin]

I've got the same issue. This happens because JSON.stringify (which is used by Mocha to produce diffs) throws on attempt to process an object with cyclic structure:

var expect = require('chai').expect;

describe('failed test with cyclic dependencies', function() {
  it('should not break Mocha', function() {
    var c = {}; c.c = c;
    expect({}).to.equal(c);
  });

  it('should not interfere with other tests', function() {
    expect(true).to.be.true;
  });
});

// Uncaught TypeError: Converting circular structure to JSON
//      at Object.stringify (native)
//      at stringify (/usr/local/lib/node_modules/mocha/lib/reporters/base.js:456:15)
//      at /usr/local/lib/node_modules/mocha/lib/reporters/base.js:184:33
//      at Array.forEach (native)
//      ...

I temporarily solved this by patching JSON.stringify, but it would be great to see the fix in Mocha. Throwing on cyclic structures is described in the spec:

If stack contains value then throw a TypeError exception because the structure is cyclical.

The most simple solution is to wrap the JSON.stringify call in a try...catch block here:

function stringify(obj) {
  if (obj instanceof RegExp) return obj.toString();
  try {
    return JSON.stringify(obj, null, 2);
  } catch (e) {
    return '' + obj;
  }
}

But it would be probably better to modify the canonicalize function so that it replaces circular references with placeholder values:

function canonicalize(obj, stack) {
  stack = stack || [];

  // if (utils.indexOf(stack, obj) !== -1) return obj;
  if (utils.indexOf(stack, obj) !== -1) return '[Circular]';

  var canonicalizedObj;

  if ('[object Array]' == {}.toString.call(obj)) {
    stack.push(obj);
    canonicalizedObj = utils.map(obj, function(item) {
      return canonicalize(item, stack);
    });
    stack.pop();
  } else if (typeof obj === 'object' && obj !== null) {
    stack.push(obj);
    canonicalizedObj = {};
    utils.forEach(utils.keys(obj).sort(), function(key) {
      canonicalizedObj[key] = canonicalize(obj[key], stack);
    });
    stack.pop();
  } else {
    canonicalizedObj = obj;
  }

  return canonicalizedObj;
}

[skeggse]

You could also use the inspect function from the util module, which takes care of pretty-printing lots of special cases including cyclic structures.

[gasi]

👍 Seeing the same issue with chai and cyclic JSON structures.

[skeggse]

Thanks!