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Exercise 9

Challenging Exercise This exercise has nothing to do with this chapter. Instead, it uses concepts you learned earlier in the book. If you can't figure out the answer, don't worry: this question can stump developers with more experience than you have.

Earlier, we learned that JavaScript has multiple versions of the numeric value zero. In particular, it has 0 and -0. While it's mathematically nonsensical to distinguish between 0 and -0, they are distinct values in JavaScript. We won't get into why JavaScript has a 0 and -0, but it can be useful in some cases.

There's a problem, however: JavaScript itself doesn't seem to realize that the values are distinct:

> 0 === -0
= true

> String(-0)
= '0'

Fortunately, you can use Object.is to determine whether a value is -0:

> let value = -0;
> Object.is(value, 0)
= false

> Object.is(value, -0)
= true

There are other ways to detect a -0 value. Without using Object.is, write a function that will return true if the argument is -0, and false if it is 0 or any other number.

What happens if you divide a non-zero integer by zero? Apply this to the problem of determining whether a value is -0.

Solution

function isNegativeZero(value) {
  return 1 / value === -Infinity;
}

This works since 1 / 0 returns Infinity and 1 / -0 returns -Infinity, thus letting us make the distinction. You can divide any other number except 0 or -0 to achieve the same result.

You can be a little more explicit with your answer as well:

function isNegativeZero(value) {
  return (value === 0) && (1 / value === -Infinity);
}

While this is a little more complex, it clearly shows that we're only interested in numbers that are 0 (or -0), which also helps eliminate unwanted division operations, which may be needed for performance reasons.

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