The global property NaN is a special value that represents an invalid number in JavaScript.
NaN stands for Not-a-Number, and it can be generated by JavaScript when you perform an operation that can’t produce a valid number.
For example, when you try to create a number from a string that has no number representation:
console.log(Number("Z"));
// NaN
console.log(Number("55"));
// 55
The same occurs when you try to parseInt() or parseFloat() values that can’t be parsed into a number:
parseInt(undefined); // NaN
parseFloat(); // NaN
In all, there are 5 conditions that will cause JavaScript to return a NaN value:
- When you parse a value with no number representation like
Number("Z") - Run a
Mathmethod that returns an invalid number likeMath.floor() - Run an arithmetic operation with invalid operands like
7 + undefined
Any arithmetic operation will result in NaN when you use an invalid value as its operand.
Except for additions between a number and a string, the following operations all return NaN values:
7 - "asd"; // NaN
3 - [1, 3, 4]; // NaN
9 + undefined; // NaN
5 * "five"; // NaN
7 + "seven"; // 7seven
To check for a NaN value, you can use the global isNaN() function provided by JavaScript.
The isNaN function returns true when you have a NaN value:
isNaN(NaN); // true
isNaN(9); // false
isNaN("zxc"); // true
The isNaN() function is required each time you want to check for a NaN value.
Using a comparison operator == or === for NaN values will always return false:
NaN == NaN; // false
NaN === NaN; // false
isNaN(1 + undefined); // true
Now you’ve learned how the NaN value works in JavaScript. Nice work! 👍