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gistfile1.txt

import {add} from "helpers"

const calculator = (a, b, op) => {
  if (op == "+") 
    return add (a, b);
}

/********************************************************************
When we unit test calculator, we are concerned with whether
calculator calls add helper or not if `op` is `+` and not with
whether calculator returns sum of `a` and `b` if op is `+`

When we test this with Jest, `add` from helpers is automatically
mocked to return undefined and we shouldn't be concerned about it.
And as it is auto mocked, we can easliy spy on it as below
*********************************************************************/

describe("Calculator", () => {
  it("should call 'add' helper, if op is '+'", () => {
    // Given
    let a = 10;
    let b = 2;
    let op = "+";
    
    // When
    const result = calculator(a, b, op);
    
    // Then
    expect(add).toBeCalledWith(a, b);
  })
});