12 July 2013
How We Handled the Problem of JS Date/Time Equality
We are using Konacha and Chai on a project and we needed to compare a bunch of time values in our specs. This post describes some of the pain we experienced testing JS Date/Time equality, and what we did about it. As a result of this work, we’ve released a Chai plugin for datetime assertions that you can find here:
###Chai-Datetime
Comparing Dates and Times in JavaScript
Date equality in JavaScript is not based on value.
var t1 = new Date(2013, 4, 30, 16, 5)
var t2 = new Date(2013, 4, 30, 16, 5)
t1 == t2 // => false
t1 === t2 // => false
t1.getTime() === t2.getTime() // => true
Example Specs (Before)
I was growing very tired of calling getTime() on my expected and actual values. Here is what the specs were looking like before we added custom assertions.
describe('time equality', function() {
it('returns true when they are the same time', function() {
var actual = new Date(2013, 4, 30, 16, 5),
expected = new Date(2013, 4, 30, 16, 5);
actual.getTime().should.equal(expected.getTime());
});
it('returns false when they are not the same time', function() {
var actual = new Date(2013, 4, 30, 16, 6),
expected = new Date(2013, 4, 30, 16, 5);
actual.getTime().should.not.equal(expected.getTime());
});
});
Not only does this seem like a lot of repetition, it also provides poor feedback when the assertion fails because we’re comparing numeric values instead of the times themselves.
describe('failing time equality', function() {
it('fails to have readable failure messages', function() {
var actual = new Date(2013, 4, 30, 16, 6),
expected = new Date(2013, 4, 30, 16, 5);
actual.getTime().should.equal(expected.getTime());
});
});
AssertionError: expected 1369944360000 to equal 1369944300000
Custom Chai Assertions
chai.Assertion.addChainableMethod('equalTime', function(time) {
var expected = time.getTime(),
actual = this._obj.getTime();
return this.assert(
actual == expected,
'expected ' + this._obj + ' to equal ' + time,
'expected ' + this._obj + ' to not equal ' + time
);
});
chai.Assertion.addChainableMethod('equalDate', function(date) {
var expectedDate = date.toDateString(),
actualDate = this._obj.toDateString();
return this.assert(
expectedDate === actualDate,
'expected ' + actualDate + ' to equal' + expectedDate,
'expected ' + actualDate + ' to not equal' + expectedDate
)
});
Example Specs (After)
Here are the specs from above with less getTime() noise and better failure messages.
Passing with less getTime() noise
describe('better time equality', function() {
it('returns true when they are the same time', function() {
var actual = new Date(2013, 4, 30, 16, 5),
expected = new Date(2013, 4, 30, 16, 5);
actual.should.equalTime(expected);
});
it('returns false when they are not the same time', function() {
var actual = new Date(2013, 4, 30, 16, 6),
expected = new Date(2013, 4, 30, 16, 5);
actual.should.not.equal_time(expected);
});
});
Failing with Better Message
describe('failing time equality', function() {
it('fails to have readable failure messages', function() {
var actual = new Date(2013, 4, 30, 16, 6),
expected = new Date(2013, 4, 30, 16, 5);
actual.should.equalTime(expected);
});
});
AssertionError: expected Thu May 30 2013 16:06:00 GMT-0400 (EDT) to equal Thu May 30 2013 16:05:00 GMT-0400 (EDT)
Date Assertions
You can also compare dates regardless of their times.
describe('date equality', function() {
it('compares dates regardless of their associated times', function() {
var actual = new Date(2013, 4, 30, 16, 6),
expected = new Date(2013, 4, 30);
actual.should.be.equalDate(expected);
});
});
Conclusion
Too much detail? Go check out the README and jump in!
