Assertions

Sinon.JS ships with a set of assertions that mirror most behavior verification methods and properties on fakes, spies and stubs.

The advantage of using the assertions is that failed expectations on fakes, spies and stubs can be expressed directly as assertion failures with detailed and helpful error messages.

Examples

Without sinon.assert

import t from "tap";
import sinon from "sinon";

t.test("without sinon.assert, failures lack helpful context", (t) => {
  const f = sinon.fake();

  // Using generic assertion without sinon.assert provides less helpful errors
  // This would fail with a generic "expected false to be true" message
  t.notOk(f.calledOnce, "fake should not be called yet");

  // Call the fake
  f();

  // Now it should be true
  t.ok(f.calledOnce, "fake should be called once");

  t.end();
});

With sinon.assert

import t from "tap";
import sinon from "sinon";

t.test("with sinon.assert, failures provide detailed error messages", (t) => {
  const msg = "Apple Pie";
  const f = sinon.fake();

  // Verify that sinon.assert throws when expectation is not met
  t.throws(
    () => sinon.assert.calledOnce(f),
    /expected fake to be called once but was called 0 times/i,
    "should throw with detailed message when fake not called"
  );

  // Call the fake
  f(msg);

  // No error should be thrown when assertions pass
  t.doesNotThrow(
    () => sinon.assert.calledOnce(f),
    "should not throw when fake called once"
  );

  t.doesNotThrow(
    () => sinon.assert.calledWith(f, msg),
    "should not throw when fake called with correct argument"
  );

  t.end();
});

Integrations

• jest-sinon
• referee-sinon - from the makers of Sinon 🙂
• sinon-chai

To make sure assertions integrate nicely with your assertion framework, you should customize sinon.assert.fail and look into sinon.assert.expose and sinon.assert.pass.