JavaScript NaN value explained

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 Math method that returns an invalid number like Math.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! 👍