Infinity and NaN

Infinity and NaN are two important values for JavaScript's (sometimes confusing) type conversion.

Infinity

In JavaScript, Infinity is of the type number:

typeof(Infinity); // "number"

Infinity is greater than any number in JavaScript.

Infinity > 99999999; // true

Negative Infinity is smaller than any number.

-Infinity < 88888888; // true

One wrinkle: division by zero surprisingly returns Infinity (not undefined).

10 / 0; // Infinity

NaN ("Not a Number")

NaN is another special number value that stands for "Not a Number". What does JavaScript consider a number? Answer: more than you think. The JavaScript Number() function can be used to view how the language will convert something to a number.

Number("cat"); // NaN
Number("67"); // 67
Number(4.3); // 4.3
Number(false); // 0

Note in that a string of alpha characters ("cat") returns NaN, but that a string of numeric characters ("67") is converted to the number 67. Also note that the boolean false converts to the number 0 (true converts to the number 1).

Most of the time, NaN's behavior is logical. Mathematical operations between any numbers and NaN will always result in NaN:

NaN + 8; // NaN
NaN / 7; // NaN
7 - NaN; // NaN

When you attempt to multiply or divide combinations of strings and numbers, NaN is the result.

"cat" * 55; // NaN
// equivalent to
NaN * 55; // NaN

It makes sense that multiplying the string "cat" by 55 should return NaN. How does it work, though? When JavaScript sees the *, it converts "cat" to NaN (using Number("cat")). Recall that any mathematical operation between numbers and NaN returns NaN.

Dividing the number 4 by the string "dog" also results in NaN:

4 / "dog"; // NaN

The string "dog" is converted to NaN (Number("dog")), and 4 / NaN returns NaN.

However, you should also be aware of JavaScript's weird behavior with NaN and type conversion. Surprisingly, dividing a boolean (true) by 5 does NOT result in NaN.

true / 5; // 0.2

JavaScript converts true to 1 (Number(true) returns 1) before performing the division. The result is numerical: 1 / 5 = 0.2.

In addition (no pun intended), adding strings and numbers does not return NaN. Rather, the result is a concatenated string: Javascript "slaughterhouse" + 5 // slaughterhouse5

Type conversion is difficult... keep practicing, you'll get used to it.

console.log(3 / (9 - 9));

console.log(1 + "farm");

console.log(1 / "farm");