Jest: Add ability to add own matchers

[mbrgm]

With plain jasmine, it is possible to add own matchers using the jasmine.addMatchers() function. Jest does not expose this functionality. Would it be possible to implement this somehow?

[gillesruppert]

This can be done already. In your package.json, add a jest configuration object and add the setupTestFrameworkScriptFile i.e.

"jest": {
  "setupTestFrameworkScriptFile": "<rootDir>/jest/customMatchers.js"
}

In this particular example, the customMatchers.js is located at project_root/jest/customMatchers.js.

In my customMatchers.js, I do the following:

// our own custom matchers
var matchers = {
  toHaveLength: function () {
    return {
      compare: function (actual, expected) {
        return {
          pass: actual.length === expected
        };
      }
    };
  }
};

jasmine.getEnv().beforeEach(function () {
  this.addMatchers(matchers);
});

That's all there is to it.

[mbrgm]

Thank you!

[jacobstr]

@gillesruppert That looks like the new syntax. Jest appears to have version 1.3 vendored.

I'm using something like this:

// Checks a react component's DOM node for matching text.
toMatchContent: function(expected) {
  return this.actual.getDOMNode().innerHTML.match(expected);
}

[wilkerlucio]

hey guys, when I try to use that I get an Object #<Object> has no method 'addMatchers', how I add matchers on the Jasmine 1.3?

[wilkerlucio]

there is any other way to that besides on a test setup file? I rather prefer load just the matchers that are relevant to a given file on that file instead of having to inject everything globally, there is a way to do that?

[seekshiva]

It seems like I have to add every package used in /jest/customMatchers.js to unmockedModulePathPatterns. Is there a way to avoid having to add each package one by one?

[seekshiva]

I'm trying to implement the matcher given in the last example in this page. That example uses clean, indent and inspect-react-element packages. Now I need to manually add each of those packages to 'unmockedModulePathPatterns'.

[cpojer]

I'd recommend doing that, yes, and then setting up the matchers in your setup env file.

[kfiron]

Can I do it programmatically? Or i must use the package.json?

[rattrayalex-stripe]

fwiw this appears to have changed in recent versions: https://facebook.github.io/jest/docs/en/expect.html#expectextendmatchers
eg; in customMatchers.js, you would now do

expect.extend({
  toHaveLength(received, argument) {
    // ...
  }
})

(assuming you are not using jasmine)

[SimenB]

Correct, this was added in https://github.com/facebook/jest/pull/1933, and is the way you're supposed to add matchers to jest (or just to expect, which works in any test test runner)