Use the toString() method to covert the Number to string, then the length() method gives you length.
Example
You can try to run the following code to learn how to get the length of a Number −
Live Demo
<!DOCTYPE html>
<html>
<body>
<script>
var num1 = 344;
var num2 = 149856;
document.write("Length of number "+num1+" = "+ num1.toString().length);
document.write("<br>Length of number "+num2+" = "+ num2.toString().length);
</script>
</body>
</html>Output
Length of number 344 = 3 Length of number 149856 = 6