30:T449,md` _In d3.express, coding with unit tests is super easy._ We start by typing a list of tests:

tests = [
  inverse(3) === 1/3,
  inverse(1) === 1,
  inverse(0) === Infinity,
  inverse(2) === 0.5
]

and display the result with an emoji:

  html([(tests.filter(d => !d).length > 0 ? '☠️' : '👌🏼')])

Of course, we went too fast, as we haven't created the function yet. So we get a \`ReferenceError: inverse is not defined\`. Now, we create the function:

  inverse = (x) => 1 - x

Oh noes, all tests have failed: ☠️

   Array(4) [false, false, false, false]

Now, we fix the function:

  inverse = (x) => 1 / x

Yay! 👌🏼

   Array(4) [true, true, true, true]

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