Conditionally mocking functions with Jest

Mocking with Jest can be really easy, and sometimes really tricky.

Recently, I wanted to mock only one specific database call in a test suite, and otherwise have my database work as usual in one of our end to end tests.

Here’s how I did it.

Defining the original function

In my setup, i want to conditionally mock the query function on my context.connection (TypeOrm connection) object.

I extract this into a constant like so:

const originalQueryImplementation = context.connection.query;

Mocking the function conditionally

With the original function saved, we can use the jest.spyOn function to mock the implementation. In my case, i want to mock that a user has “swiped” an average of 1000 swipes daily recently.

const originalQueryImplementation = context.connection.query;
jest.spyOn(context.connection, 'query').mockImplementation((sql, ...args) => {
      if (sql.includes('average_swipes')) {
        return Promise.resolve([{ average_swipes: 1000 }]);
      }
      return originalQueryImplementation.apply(context.connection, [sql, ...args]);
    });

Notice how i can conditionally check the contents of the arguments, to not interfere with other calls to query. If the aguments to not match, i can apply the captured arguments onto the original function implementation and return. I also pass in the original connection object as the this context to the original function.

Yay!