How to use sinon to replace the same method with two different return values?

Question

A method that I am unit testing calls the same helper method with different parameters multiple times within the method. In order for me to test my method, I want to use sinon's replace function to replace the return value of this helper method with my mock data, but I need each time I call that helper method to return different mock data. How can I do this?

As an example:

const obj = {
  foo(num) {
    return 5 + num;
  },
  methodToTest() {
    return foo(1) + foo(2);
  },
};

I want to test whether methodToTest() will work properly if foo returns 6 when it is called with parameter value 1, and foo returns 7 when called with parameter value 2.

I guess what I'm looking for is a way to replace foo's return value depending on the parameter passed in, something like:

   sinon.replace(obj, 'foo(1)', sinon.fake.returns(6));
   sinon.replace(obj, 'foo(2)', sinon.fake.returns(7));

Any idea how I could do this? Would be very much appreciated.

Answer 1

Just create a fake foo function which provides multiple return values dynamicly based on argument.

E.g.

index.js:

const obj = {
  foo(num) {
    return 5 + num;
  },
  methodToTest() {
    return this.foo(1) + this.foo(2);
  },
};

module.exports = obj;

index.test.js:

const obj = require('./');
const sinon = require('sinon');

describe('68463040', () => {
  it('should pass', () => {
    function fakeFoo(arg) {
      if (arg == 1) {
        return 6;
      }
      if (arg == 2) {
        return 7;
      }
    }
    sinon.replace(obj, 'foo', fakeFoo);
    const actual = obj.methodToTest();
    sinon.assert.match(actual, 13);
  });
});

test result:

  68463040
    ✓ should pass


  1 passing (3ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |      75 |      100 |      50 |      75 |                   
 index.js |      75 |      100 |      50 |      75 | 3                 
----------|---------|----------|---------|---------|-------------------