[Testing] JavaScript Mocking Fundamentals

Often when writing JavaScript tests and mocking dependencies, you’ll want to verify that the function was called correctly. That requires keeping track of how often the function was called and what arguments it was called with. That way we can make assertions on how many times it was called and ensure it was called with the right arguments.

 

Function to be mocked: utils.js

// returns the winning player or null for a tie
// Let's pretend this isn't using Math.random() but instead
// is making a call to some third party machine learning
// service that has a testing environment we don't control
// and is unreliable so we want to mock it out for tests.
function getWinner(player1, player2) {
  const winningNumber = Math.random();
  return winningNumber < 1 / 3
    ? player1
    : winningNumber < 2 / 3
      ? player2
      : null;
}

module.exports = {getWinner};

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