Test for the custom Promise implementation for codefrontend.com article

Futurable.spec.ts

import Futurable from "./Futurable";

describe("Futurable <constructor>", () => {
  it("returns a promise-like object, that resolves it's chain after invoking <resolve>", (done) => {
    new Futurable<string>((resolve) => {
      setTimeout(() => {
        resolve("testing");
      }, 20);
    }).then((val) => {
      expect(val).toBe("testing");
      done();
    });
  });

  it("is always asynchronous", () => {
    let value = "no";
    new Futurable<string>((resolve) => {
      value = "yes;";
      resolve(value);
    });
    expect(value).toBe("no");
  });

  it("resolves with the returned value", (done) => {
    new Futurable<string>((resolve) => resolve("testing")).then((val) => {
      expect(val).toBe("testing");
      done();
    });
  });

  it("resolves a Futurable before calling <then>", (done) => {
    new Futurable<string>((resolve) =>
      resolve(new Futurable((resolve) => resolve("testing")))
    ).then((val) => {
      expect(val).toBe("testing");
      done();
    });
  });

  it("resolves a Futurable before calling <catch>", (done) => {
    new Futurable<string>((resolve) =>
      resolve(new Futurable((_, reject) => reject("fail")))
    ).catch((reason) => {
      expect(reason).toBe("fail");
      done();
    });
  });

  it("catches errors from <reject>", (done) => {
    const error = new Error("Why u fail?");

    new Futurable((_, reject) => {
      return reject(error);
    }).catch((err: Error) => {
      expect(err).toBe(error);
      done();
    });
  });

  it("catches errors from <throw>", (done) => {
    const error = new Error("Why u fail?");

    new Futurable(() => {
      throw error;
    }).catch((err) => {
      expect(err).toBe(error);
      done();
    });
  });

  it("does not change state anymore after promise is fulfilled", (done) => {
    new Futurable((resolve, reject) => {
      resolve("success");
      reject("fail");
    })
      .catch(() => {
        done.fail(new Error("Should not be called"));
      })
      .then((value) => {
        expect(value).toBe("success");
        done();
      });
  });

  it("does not change state anymore after promise is rejected", (done) => {
    new Futurable((resolve, reject) => {
      reject("fail");
      resolve("success");
    })
      .then(() => {
        done.fail(new Error("Should not be called"));
      })
      .catch((err) => {
        expect(err).toBe("fail");
        done();
      });
  });
});

describe("Futurable chaining", () => {
  it("resolves chained <then>", (done) => {
    new Futurable<number>((resolve) => {
      resolve(0);
    })
      .then((value) => value + 1)
      .then((value) => value + 1)
      .then((value) => value + 1)
      .then((value) => {
        expect(value).toBe(3);
        done();
      });
  });

  it("resolves <then> chain after <catch>", (done) => {
    new Futurable<number>(() => {
      throw new Error("Why u fail?");
    })
      .catch(() => {
        return "testing";
      })
      .then((value) => {
        expect(value).toBe("testing");
        done();
      });
  });

  it("catches errors thrown in <then>", (done) => {
    const error = new Error("Why u fail?");

    new Futurable((resolve) => {
      resolve();
    })
      .then(() => {
        throw error;
      })
      .catch((err) => {
        expect(err).toBe(error);
        done();
      });
  });

  it("catches errors thrown in <catch>", (done) => {
    const error = new Error("Final error");

    new Futurable((_, reject) => {
      reject(new Error("Initial error"));
    })
      .catch(() => {
        throw error;
      })
      .catch((err) => {
        expect(err).toBe(error);
        done();
      });
  });

  it("short-circuits <then> chain on error", (done) => {
    const error = new Error("Why u fail?");

    new Futurable(() => {
      throw error;
    })
      .then(() => {
        done.fail(new Error("Should not be called"));
      })
      .catch((err) => {
        expect(err).toBe(error);
        done();
      });
  });

  it("passes value through undefined <then>", (done) => {
    new Futurable((resolve) => {
      resolve("testing");
    })
      .then()
      .then((value) => {
        expect(value).toBe("testing");
        done();
      });
  });

  it("passes value through undefined <catch>", (done) => {
    const error = new Error("Why u fail?");

    new Futurable((_, reject) => {
      reject(error);
    })
      .catch()
      .catch((err) => {
        expect(err).toBe(error);
        done();
      });
  });
});

describe("Futurable <finally>", () => {
  it("it is called when Futurable is resolved", (done) => {
    new Futurable((resolve) => resolve("success")).finally(() => {
      done();
    });
  });

  it("it is called when Futurable is rejected", (done) => {
    new Futurable((_, reject) => reject("fail")).finally(() => {
      done();
    });
  });

  it("it preserves a resolved promise state", (done) => {
    let finallyCalledTimes = 0;

    new Futurable((resolve) => resolve("success"))
      .finally(() => {
        finallyCalledTimes += 1;
      })
      .then((value) => {
        expect(value).toBe("success");
        expect(finallyCalledTimes).toBe(1);
        done();
      });
  });

  it("it preserves a rejected promise state", (done) => {
    let finallyCalledTimes = 0;

    new Futurable((_, reject) => reject("fail"))
      .finally(() => {
        finallyCalledTimes += 1;
      })
      .catch((reason) => {
        expect(reason).toBe("fail");
        expect(finallyCalledTimes).toBe(1);
        done();
      });
  });
});