assert.calledOn(spyOrSpyCall, object);

Passes, when the fake, spy or stub was called at least once with object as this.

It's possible to assert on a dedicated spy call: sinon.assert.calledOn(call, arg1, arg2, ...);.

import * as sinon from "sinon";
const spy = sinon.spy();
const object = {
  name: "Apple Pie"
};

// Has not been called yet, so the error message is a bit short :)
sinon.assert.calledOn(spy, object);
// => Uncaught: Error [AssertError]: expected spy to be called with { name: 'Apple Pie' } as this but was called with

spy.call(object);

// Generates no error
sinon.assert.calledOn(spy, object);

// Generates no error
sinon.assert.calledOn(spy.firstCall, object);

Asserting on a spyCall

import * as sinon from "sinon";
const spy = sinon.spy();
const object = {
  name: "Apple Pie"
};

spy.call(object);

// get a spyCall instance
const call = spy.firstCall;

// Generates no error
sinon.assert.calledOn(call, object);

See Function.prototype.call().

Example using test framework

import tap from "tap";
import * as sinon from "sinon";

tap.test(
  "assert.calledOn - passes when spy was called with correct this context",
  (t) => {
    const obj = { name: "test object" };
    const fake = sinon.fake();

    fake.call(obj);

    t.doesNotThrow(() => {
      sinon.assert.calledOn(fake, obj);
    }, "assertion should pass");

    t.end();
  }
);

tap.test("assert.calledOn - fails when spy was not called", (t) => {
  const obj = { name: "test object" };
  const fake = sinon.fake();

  t.throws(
    () => sinon.assert.calledOn(fake, obj),
    /expected fake to be called with/,
    "assertion should fail when not called"
  );

  t.end();
});

tap.test(
  "assert.calledOn - fails when spy was called with different this context",
  (t) => {
    const obj1 = { name: "obj1" };
    const obj2 = { name: "obj2" };
    const fake = sinon.fake();

    fake.call(obj1);

    t.throws(
      () => sinon.assert.calledOn(fake, obj2),
      /expected fake to be called with/,
      "assertion should fail with wrong this context"
    );

    t.end();
  }
);