How to Test Errors with Jest in Typescript

Leverage Jest to assess errors in your Typescript development

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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 test is to edit the calculator class and add the following steps.

  • Define array with the list of valid actions.

  • If the Action is not in the array throw an error.

Your code will look like this:

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

            throw new Error("Invalid action");

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

        return  {
            mathAction: action,


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

  • Use a try-catch statement.

  • call the generateCalcSettings with the wrong parameters

  • The catch takes the return 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 {
        } catch (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
    ✓ 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 that the error message is expected.