#### Defining the `jest` spec
A "spec" is just a test file. We call it "spec",
short for "specification", because well written
tests clearly define the behavior of a function.

There is no need for comments or guessing. The function
does exactly what the spec says it will do.
This also means we can change the inner 
implementation of the function at any 
time as long as it still meets the spec.

The test cases in the spec run against
your code to ensure this.

To define a test case, we need to define
the input value and the output value
we are validating the function against.

Here are some simple tests that
confirm the `filters` parameter
is converted to the type required
by the search feature. I'm essentially
taking the example I gave before but
writing it in code.

```typescript
describe('replaceMultiValueFilters', () => {

  // this tests what we were discussing earlier
  it('replaces one filter', () => {
    expect(replaceMultiValueFilters(
      'id:[id1,id2,id3,id4]'
    )).toEqual(
      '(id:id1 OR id:id2 OR id:id3 OR id:id4)'
    );
  });

  // we'll add to more tests below just for
  // good measure
  
  it('preserves other filters', () => {
    expect(replaceMultiValueFilters(
      'filter2:value AND id:[id1,id2,id3,id4]'
    )).toEqual(
      'filter2:value AND (id:id1 OR id:id2 OR id:id3 OR id:id4)'
    );
  });
  
  it('replaces multiple', () => {
    expect(replaceMultiValueFilters(
      'filter2:[value,value2] AND id:[id1,id2,id3,id4]'
    )).toEqual(
      '(filter2:value OR filter2:value2) AND (id:id1 OR id:id2 OR id:id3 OR id:id4)'
    );
  });
});
```

With our tests defined, we can now see clearly how
this function is intended to work.

Defining the jest spec

Defining the `jest` spec