How to pass a custom error message to a jasmine matcher?

If you take a look at the jasmine source code you will see that there is no way to set the message from outside a matcher. For example the toBeNaN matcher.

/**
 * Matcher that compares the actual to NaN.
 */
jasmine.Matchers.prototype.toBeNaN = function() {
  this.message = function() {
      return [ "Expected " + jasmine.pp(this.actual) + " to be NaN." ];
  };

  return (this.actual !== this.actual);
};

As you can see the messages is hard coded into the matcher and will be set when you call the matcher. The only way I can think off to have your own messages is to write your matcher like described here

This issue is tracking interest in implementing custom error messages using a .because() mechanism.

In the meantime, avrelian has created a nice library which implements custom error messages using a since() mechanism – jasmine-custom-message.

Yes, it can be done.

You may define a custom matcher in global scope, overriding the error message in jasmine as below:

beforeEach(function () {
    jasmine.addMatchers({
        toReport: function () {
            return {
                compare: function (actual, expected, msg) {
                    var result = {pass: actual == expected};
                    result.message = msg;
                    return result;
                }
            }
        }
    });
});

Update 2022; Use .withContext(...) method instead of below (as optional parameter is deprecated).

---

Jasmine already supports optional parameter in all matchers (toBe, toContain, and others), so you can use:

expect(true).toBe(false, 'True should be false').

Then in output it will look like this:

Message:
    Expected true to be false, 'True should be false'.

Link to commit (this is not described in documentation): https://github.com/ronanamsterdam/DefinitelyTyped/commit/ff104ed7cc13a3eb2e89f46242c4dbdbbe66665e