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

Question

In all the testing frameworks I have used, there is an optional parameter to specify you own custom error message.

This can be very useful, and I can't find a way to do this out of the box with jasmine.

I've had 3 other developers ask me about this exact functionality, and when it comes to jasmine I don't know what to tell them.

Is it possible to specify your own custom error message on each assertion ?

Answer 1

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

Answer 2

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