Dany Paredes
Danywalls | Angular ♥ Web


Danywalls | Angular ♥ Web

How test errors with Jest in Javascript

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How test errors with Jest in Javascript

Use Jest to test errors in your code

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Dany Paredes
·Jan 23, 2021·

2 min read

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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 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) {

            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 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 {
        } 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 the error message is expected.

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