Jest (Received function did not throw, Error handling)

Question

I now have a problem with Jest handling Error.

My code:

export class ResistorColor {
  private colors: string[];

  constructor(colors: string[]) {
    this.colors = colors;
  }
  Obj: any = {
    black: "0",
    brown: "1",
    red: "2",
    orange: "3",
    yellow: "4",
    green: "5",
    blue: "6",
    violet: "7",
    grey: "8",
    white: "9",
  };
  value = (): number => {
    let result = "";
    if (this.colors.length < 2) {
      throw new Error("At least two colors need to be present");
    }
    this.colors.length = 2;
    for (let a = 0; a < this.colors.length; a++) {
      for (let i in this.Obj) {
        if (this.colors[a] === i) {
          result += this.Obj[i];
        }
      }
    }
    return parseInt(result);
  };
}

my test file is:

import { ResistorColor } from "./resistor-color-duo";

describe("Resistor Colors", () => {
  it("Brown and black", () => {
    const resistorColor = new ResistorColor(["brown", "black"]);
    expect(resistorColor.value()).toEqual(10);
  });

  it("Blue and grey", () => {
    const resistorColor = new ResistorColor(["blue", "grey"]);
    expect(resistorColor.value()).toEqual(68);
  });

  it("Yellow and violet", () => {
    const resistorColor = new ResistorColor(["yellow", "violet"]);
    expect(resistorColor.value()).toEqual(47);
  });

  it("Orange and orange", () => {
    const resistorColor = new ResistorColor(["orange", "orange"]);
    expect(resistorColor.value()).toEqual(33);
  });

  it("Ignore additional colors", () => {
    const resistorColor = new ResistorColor(["green", "brown", "orange"]);
    expect(resistorColor.value()).toEqual(51);
  });

  it("Throws error when not enough colors", () => {
    expect(() => new ResistorColor(["green"])).toThrowError(Error);
  });
});

I've passed 5 cases until the last case about throwing an Error.

It's should throw an Error while I'm checking in If statement.

I've read:

JEST Received function did not throw, but HTTPError is thrown

Error is thrown but Jest's `toThrow()` does not capture the error

But it seems not likely my problem.

and then I received.

expect(received).toThrowError(expected)

    Expected constructor: Error

    Received function did not throw

I'm new to typescript and Jest, any helps would be Appreciated.

Answer 1

It would be hard for Jest to not notice an error, the problem is that it is really not thrown. In this case it's thrown not in constructor but in value , which is not called.

It should be:

const resistorColor = new ResistorColor(["green"]);
expect(() => resistorColor.value()).toThrowError(Error);