Print message on expect() assert failure

Is there a way to print a custom error message when a Jasmine expect() fails?

As an example, for end to end testing I have an array of web pages and I use one test to go to each URL and assert an element exists on each page. I know I can put every expect() into a separate test, but I'd rather iterate through the array and log the page URL on failure.

Solution 1:

UPDATE

I see people still are finding this. Later information from the Jasmine team is that there is an undocumented feature on the expect - you can include a custom failure message and it just works:

expect( fields[i].element.exists() ).toEqual(true, field[i].name + ' is expected to exist');

Which is exactly what I was originally looking for.

Original answer follows:

I've been looking for exactly this today, and put a comment here: https://github.com/adobe/brackets/issues/2752

The syntax that has been discussed is an extension to Jasmine to permit a because to be added - so you'd be able to write:

expect( fields[i].element.exists() ).toEqual(true).because( field[i].name + 'is expected to exist');

That is still being discussed after a few years, and may not come to fruition. Another way that I've found to do this is to create a custom matcher. In general I think I'd discourage a custom matcher without being sure you're covering all the bases with it, but in this case we're really checking a true/false value, so the matcher isn't too scary.

We can create the custom matcher with a beforeEach:

beforeEach(function() {
  var matchers = {
    toEqualBecause: function( value, message ) {
      this.message = function() {
        return "Expected '" + this.actual + "' to equal '" + value + "' because " + message;  
      };

      return this.actual == value;  
    }
 };

  this.addMatchers(matchers);
});

We can then use this matcher to put a message with our failures as such:

expect( field[i].element.exists() ).toEqualBecause( true, field[i].name );

Which will give a failure output including the field name as such:

Expected 'false' to equal 'true' because account_name

Solution 2:

Yes, we can print a custom error message when an expect() failed in Jasmine.

 Code Snippet:

  it('print a custom error message when an expect failed', function() {

    var elemenToBeDisplayed=element(by.css("userName"));

    /*custom error message will be displayed if expected condition 
    failed*/

    expect(elemenToBeDisplayed.isPresent).toBe(true,'Write your custom       
         error message here');
   });

Solution 3:

Jasmine 3.3 includes withContext as the officially supported way to specify additional information about the expectation without worrying about which matcher you are using.

Solution 4:

Since Jasmine 3.3, there's a way to do it through withContext

Example:

expect(someValue).withContext('expected someValue to be true...').toBe(true)

See also https://github.com/jasmine/jasmine/issues/641#issuecomment-457037665

Solution 5:

The other answers explain how to hack 'expect', but there is another approach that may solve your problem, though it requires you to flip your thinking around a little bit. Instead of thinking of the 'expect' as your behavior under test, think of all the expectations under a single 'it' call as your behavior under test.

The case where I've come across this problem the most is when I have a function that is doing some kind of intensive parsing and I want to write 20, nearly identical, tests.

Arrange your inputs and outputs like so:

var testDatas = [
  {
    input: 'stringtoparse1',
    output: 'String To Parse 1'
  },
  {
    input: 'stringtoparse2',
    output: 'String To Parse 2'
  },
  {
    input: 'stringtoparse3',
    output: 'String To Parse 3'
  },
];

Now iterate over the list of your test data, and call 'it' from inside the loop like so:

testDatas.forEach(function(test) {
  it('should parse for input ' + test.input, function() {
    expect(myParser(test.input).toEqual(test.output);
  });
});

You get to reduce the amount of extraneous code flying around your tests and you get to format a message for each expectation, or group of expectations.