expect.toThrowWithCause.js

function throwme() {
  const err = new Error("foo");
  err.cause = "bar";
  throw err;
};

const expect = (fn) => {
  const self = {};
  self.fn = fn;
  let thrown = false
    try {
      self.res = self.fn.call();
    } catch (e) {
      self.res = e;
      self.thrown = true;
    }
    return {
      toThrowWithCause: (c) => {
        if (self.thrown !== true) {
          throw new Error(`Expected ${self.fn} to throw, but it did not!`)
        }
        console.dir(Object.getOwnPropertyDescriptors(c));
        console.dir(Object.getOwnPropertyDescriptors(self.res));
        console.log('===========')
        if (self.res instanceof Error && String(self.res.cause) === String(c)) {
          return true;
        }
        throw new Error(`Expected ${String(c)}, recieved ${String(self.res.cause)}`)
    }
  }
};

const expected = 'foo'