Deep Diff Test Failures in Karma with Mocha - mocha.js

Question

I am running the following test:
describe("objects", function () {
it("should equal", function () {
var a = {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
x: 3
}
}
};
var b = {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
x: 4
}
}
};
a.should.deep.equal(b);
});
});
The test fails as expected, but the error message is not at all helpful.
AssertionError: expected { Object (a, b, ...) } to deeply equal { Object (a, b, ...) }
How would I get it so that it outputs a prettified json comparison instead?
Libraries I am currently using:
karma 0.12.1
karma-mocha 0.1.6
karma-mocha-reporter 0.3.0
karma-chai 0.1.0

Answer 1

You can change where the message gets truncated with the following:
chai.config.truncateThreshold = 0
So for your example:
var chai = require('chai');
var expect = chai.expect;
chai.config.truncateThreshold = 0;
describe.only("objects", function () {
it("should equal", function () {
var a = {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
x: 3
}
}
};
var b = {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
x: 4
}
}
};
expect(a).to.deep.equal(b);
});
});
Which will result in:
AssertionError: expected { a: 1, b: 2, c: { a: 1, b: 2, c: { a: 1, b: 2, x: 3 } } } to deeply equal { a: 1, b: 2, c: { a: 1, b: 2, c: { a: 1, b: 2, x: 4 } } }
+ expected - actual
"b": 2
"c": {
"a": 1
"b": 2
+ "x": 4
- "x": 3
}
}
}

Answer 2

The mocha reporter has an option for this: showDiff: true
You can stick this in your karma config:
config.set({
frameworks: [ 'mocha', 'chai', ... ],
plugins: [
require('karma-mocha'),
require('karma-chai'),
require('karma-mocha-reporter'),
...
],
reporters: [ 'mocha', ... ],
mochaReporter: {
showDiff: true, // <-- This!
},
...
});

Answer 3

You can use assert-diff library to show only the difference when test fails.
var assert = require('assert-diff');
describe("objects", function () {
it("should equal", function () {
var a = {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
x: 3
}
}
};
var b = {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
c: {
a: 1,
b: 2,
x: 4
}
}
};
assert.deepEqual(a, b);
});
});
this will give you an error with the difference
1) objects should equal:
AssertionError: { a: 1, b: 2, c: { a: 1, b: 2, c: { a: 1, b: 2, x: 3 } } } deepEqual { a: 1, b: 2, c: { a: 1, b: 2, c: { a: 1, b: 2, x: 4 } } }
{
c: {
c: {
- x: 4
+ x: 3
}
}
}