How to add a custom assertion to should.js

customAssert.js

var should = require('should');

should.use(function (should, Assertion) {
  Assertion.add('myEql', function (value, description) {
    this.params = { operator: 'to be' + should.format(expectedTraces), message: description};

    this.assert(value === this.obj);
  });
});

describe('test', function () {
  it('should work', function () {
    var a = 42;
    a.should.be.myEql(42);
  });
});

in the assertion function this.obj is your original value ( var a in our case ) and value is the parameter of myEql(). The only thing left is to call this.assert for wathever condition you can need.