I have a custom matcher myMatcher containing a function to be called from the matcher:

expect.extend({

  someFunction(x,y) {
     return this.equals(....) ....
  },

  myMatcher(received, expected) {
      ...
      this.someFunction(x,y);
  }

});

But this fails:

 TypeError: this.someFunction is not a function

How can I define a function within a matcher? Within the function this needs to be the same as within myMatcher.