Prerequisites

• Checked that your issue isn't already filed by cross referencing issues with the common mistake label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
• 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with:
  node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend avoiding the use of globally installed Mocha.

Description

Mocha prints nice diffs for failed assertions. It'd be great to show these diffs when running mocha tests in IDE.
Currently, IDE accesses err object directly to format error info: https://github.com/JetBrains/mocha-intellij/blob/3c40655fcb8e4b216586d19ab83cac8c3f98e4a9/lib/mochaIntellijReporter.js#L157-L176 and doesn't print diffs in console (only clicking "Click to see difference" opens a separate diff dialog).

What do you think about allowing IDE to show unified diffs for failed assertions? // cc @boneskull

A possible API might look this:

// lib/utils.js

exports.prettifyError = function (err) {
  return {
    msg: '<colored text with unified diff>',
    stack: '<colored tail of stack>'
  };
};

In this case, both list function from lib/reporters/base.js and IDE may use prettifyError.

Original issue in WebStorm's tracker: https://youtrack.jetbrains.com/issue/WEB-28905