---
title: "How to Test Errors with Jest in Typescript"
date: "2021-01-23"
slug: "how-to-test-errors-with-jest-in-typescript"
author: "Dany Paredes"
canonical: "https://danywalls.com/how-to-test-errors-with-jest-in-typescript"
description: "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 err..."
---

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:

```javascript
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 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:

```typescript
    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

```bash
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 that the error message is expected.

Nice!