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 action.
- 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, update the Calculator.test.ts 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 wrong parameters
- The catch take the return and use tobeInstanceOf and compare 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
dany@dany:~/Desktop/calculator(master)$ npm run test
> calculator@1.0.0 test /home/dany/Desktop/calculator
> jest
PASS src/tests/Calculator.test.ts
Calculator
✓ should return a number (1 ms)
✓ should increment the number (1 ms)
✓ should return a calculation Object logic (1 ms)
✓ should return a error if is invalid action (1 ms)
Test Suites: 1 passed, 1 total
Tests: 4 passed, 4 total
Snapshots: 0 total
Time: 2.253 s
Ran all test suites.
Great! The matcher toBeInstanceOf allow to get the error type and also read the properties message to validate the error message is the expected.
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