I'm continuing with Jest and typescript, we already know how to expect and assert variables and objects in our code, next step is handling errors.

The Calculator has a new requirement, if the type of calculation is not +,- or / we need to throw an error.

The lest to edit the calculator class and add the following steps.

• Define array with the list of valid actions.
• If the Action is not into the array throw an error.

Your code will look like:

export class Calculator {
    static validActions: Array<string> = ['+',"-","/"];
    public static increase(value: number) {
        return value + 1;
    }
    public static generateCalcSetting(valueA: number, action: string, valueB: number) {

        if(!this.validActions.includes(action)){
            throw new Error("Invalid action");
        }

        let result : number;
        switch (action) {
            case "+":
                result = valueA + valueB;
                break;
            case "-":
                result = valueA - valueB;
                break;
            case "/":
                result = valueA / valueB;
            default:
                result = 0;
        }


        return  {
            valueA,
            mathAction: action,
            valueB,
            result
        }
    }

 }

Perfect, create the Calculator.test.ts file and use jest matchers toBeInstanceOf() it helps to get the type, so we need to do the following changes.

• Use try catch statement.
• call the generateCalcSettings with the wrong parameters
• The catch takes the return and uses tobeInstanceOf and compares with type Error.
• compare the error.message toBe ('Invalid action')

Your code should look like:

    it('should return a error if is invalid action',() => {

        try {
            Calculator.generateCalcSetting(1,'M',5);    
        } catch (error) {
            expect(error).toBeInstanceOf(Error);
            expect(error.message).toBe('Invalid action');
        }

    })

Run our tests and get the results

npm run test
> calculator@1.0.0 test /home/dany/Desktop/calculator
> jest

 PASS  src/tests/Calculator.test.ts
  Calculator
    ✓ should return an error if is invalid action (1 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        2.253 s
Ran all test suites.

Great! The matcher toBeInstanceOf allows us to get the error type and also read the properties message to validate the error message is expected.