Objects prematurely coerced into string primitives

[oliversalzburg]

This is regarding an issue I reported on the WebStorm tracker: https://youtrack.jetbrains.com/issue/WEB-23383

Differences between symbols that are not strictly equal, appear as being equal and are thus hard to read and confusing.

Here is a simplified version of a test where I noticed this behavior:

"use strict";

const chai           = require( "chai" );
const expect         = chai.expect;

chai.config.truncateThreshold = 0;

describe( "Strings", () => {
    it( "should correctly display differences", () => {
        expect( new String( "foo" ) ).to.equal( "foo" );
    } );
} );

Because new String( "foo" ) is not equal to the primitive "foo", the test will correctly fail. However, in the resulting output, the difference is not immediately apparent.

In this simple test, the AssertionError will pretty clearly identify the issue. However, in larger objects, this can be much harder to identify, especially if the truncation threshold is not raised.

[Munter]

How assertion libraries generate error messages is outside of the scope of Mocha. I suggest taking this up with chai or the relevant chai plugin you might be using

[oliversalzburg]

@Munter Apparently the behavior is related to https://github.com/mochajs/mocha/blob/master/lib/utils.js#L435

Also, @boneskull suggested to post this here

[papandreou]

Quite reproducible:

it('should foo', function () {
    var err = new Error('oh no');
    err.actual = new String('foo');
    err.expected = 'foo';
    throw err;
});

Output:

$ mocha foo.js 


  1) should foo

  0 passing (8ms)
  1 failing

  1)  should foo:

      Error: oh no
      + expected - actual


      at Context.<anonymous> (foo.js:2:15)

[boneskull]

@Munter Yeah, the problem seems to be in our diffing code; Chai actually outputs the difference correctly. See original issue.

[oliversalzburg]

Does the fix also apply to new Number(42)? Because the message explicitly states "String".