Implement _check

[mahim-18]

Solved the "good first issue" #1

Changes Made :

Made changes in calculator.test.js
describe('_check', () => { beforeEach(() => { sinon.spy(calculator, '_check'); });
This is done so that all the tests can run at a same time

Moved the repeating function for checking the TypeError once globally and using it at all the checks.

This helped me gain some knowledge about how such tests are made for checking if all functionalities are working as desired.

[dantharejabot]

Thanks for the submission, @mahim-18.

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