Returning Consecutive Values in Jasmine

August 10, 2014

spyOnAndReturnConsecutive(foo, 'getBar', [1, 2, 3]);

expect(foo.getBar()).toEqual(1);
expect(foo.getBar()).toEqual(2);
expect(foo.getBar()).toEqual(3);
expect(foo.getBar()).toEqual(3);

Jasmine provides the ability to stub methods and return specific values. You can even return different values based on the arguments. So how do we get the snippet above to work?

You can leverage Jasmine’s ` andCallFake ` and ` andReturn ` to modify the return value every time the stub is called. Drop the following function definitions into ` spec_helper.js ` and start stubbing!

function spyOnAndReturnConsecutive(object, method, values) {
  spyOn(object, method)
    .andCallFake(returnConsecutive(object, method, values));
}

function returnConsecutive(object, method, values) {
  var nextValue = values.shift();

  return function () {
    if (values.length > 1) {
      object[method]
        .andCallFake(returnConsecutive(object, method, values));
    } else {
      object[method].andReturn(values[0]);
    }

    return nextValue;
  };
}

How does it work?

Let’s take a look at the first function:

function ... {
  spyOn(object, method)
    .andCallFake(returnConsecutive(object, method, values));
}

This simply creates the initial spy on the object and returns the result of our other function - ` returnConsecutive `.

The ` returnConsecutive ` function does two things:

returns the next value
modify what the stub returns

This is what it looks like with just returning the next value:

function ... {
  var nextValue = values.shift();

  return function () {
    ...

    return nextValue;
  };
}

Now let’s take a look at where the chaining is happening:

function ... {
  ...

  return function () {
    if (values.length > 1) {
      object[method]
        .andCallFake(returnConsecutive(object, method, values));
    } else {
      object[method].andReturn(values[0]);
    }

    ...
  };
}

If there are more than one value that still needs to be returned, we modify the stub to call ` returnConsecutive ` again to return the next set of values.

If there is only a single value left to be returned, we set the stub to statically return the last value using ` andReturn `. This means the stub will keep returning the last value in the array after all of them have been returned.

Full Example

'use strict';

describe('foo', function () {

  var foo;

  beforeEach(function () {
    foo = {
      getBar: function () {
        return -1;
      }
    };
  });

  it('returns consecutive values', function () {
    spyOnAndReturnConsecutive(foo, 'getBar', [1, 2, 3]);

    expect(foo.getBar()).toEqual(1);
    expect(foo.getBar()).toEqual(2);
    expect(foo.getBar()).toEqual(3);
    expect(foo.getBar()).toEqual(3);
  });

  // Helper functions

  function spyOnAndReturnConsecutive(object, method, values) {
    spyOn(object, method)
      .andCallFake(returnConsecutive(object, method, values));
  }

  function returnConsecutive(object, method, values) {
    var nextValue = values.shift();

    return function () {
      if (values.length > 1) {
        object[method]
          .andCallFake(returnConsecutive(object, method, values));
      } else {
        object[method].andReturn(values[0]);
      }

      return nextValue;
    };
  }

});

Jan Rubio

Returning Consecutive Values in Jasmine

How does it work?

Full Example

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