A Weird Thing with NaN

At work we encountered a problem with NaN. For those who do not know, NaN stands for not-a-number. It represents when a computer cannot get a number from a mathematical operation.

Here are some different situations with JavaScript that can result in NaN:

• Creating a number too big for the computer.
• Dividing a number by a string.
• Failing to convert a string to a number.
• Doing an operation with NaN.

Okay, back to the bug.

We were trying to validate some user input. The input comes from a URL query, which can look like this:

example.com/search?q=123

The query comes into our system as a string, even if it only contains digits. For those that do not know, a string is just something that represents text. Digits can be formatted as a number or string. 123 would be a number and "123" would be a string. They are not equal to each other.

Later in the system we try to convert the input to a number. And then check if it is actually a number. But this check wasn’t working, it was always returning true. Even for inputs like "hello there" which is clearly not a number.

let input = "hello there";
let value = Number(input);
if (typeof value === "number")…