Be Careful With Truthy and Falsy Values

When using truthy/falsy evaluations in JavaScript extra care must be taken.

Take the following example. The function's intention is to print a message based on how many cars a user has. If no parameter is passed in, it means that the number of cars is not known.

function printNumCarMsg(numCars) {
  console.log(
    numCars ? `I have ${numCars} car(s).` : "I don't know how many cars I have."
  );
}

Testing the function with 2 given as numCars and without any parameters produces the following. So far so good.

printNumCarMsg(2);
// I have 2 car(s).

printNumCarMsg();
// I don't know how many cars I have.

Now, what if a user does not have any cars? Passing in 0 has the following result. What happened? We do know how many cars there are - 0. So why is this the result?

printNumCarMsg(0);
// I don't know how many cars I have.

Look at the expression numCars ? `I have ${numCars} car(s).` : "I don't know how many cars I have.". The left side of this expression is a truthy expression. Meaning, 0 would evaluate to false. The function needs to be modified like so:

function printNumCarMsg(numCars) {
  console.log(
    typeof numCars !== 'undefined'
      ? `I have ${numCars} car(s).`
      : "I don't know how many cars I have."
  );
}

Now we're in business.

printNumCarMsg(0);
// I have 0 car(s).

The lesson here is to be careful when using truthy/falsy values. This isn't limited to expressions, of course. It can happen anywhere you evaluate the value, like if statements.