In JavaScript, You can use any of the following methods to check if a variable is a number or not.

  1. Using isNaN() function.
  2. Using Number().
  3. Using typeof().

1) Using isNaN()

This function returns true if the variable is not a number.

<!DOCTYPE html>
<html>
<head>
<title>Learn JavaScript</title>
</head>
<body>
<script>
var a=50, b="Hello";
if(isNaN(a))
{
document.write(a + " (a) Is not a number<br>");
}
else
{
document.write(a + " (a) Is a number<br>");
}

if(isNaN(b))
{
document.write(b + " (b) Is not a number<br>");
}
else
{
document.write(b + " (b) Is a number<br>");
}
</script>
</body>
</html>
//Output
50 (a) Is a number
Hello (b) Is not a number

2) Using Number()

This function returns true if the variable is a number.

<script>
var a=50, b="Hello";
if(Number(a)){
document.write(a + " (a) Is a number<br>");
}
else{
document.write(a + " (a) Is not a number<br>");
}

if(Number(b)){
document.write(b + " (b) Is a number<br>");
}
else{
document.write(b + " (b) Is not a number<br>");
}
</script>
//Output
50 (a) Is a number
Hello (b) Is not a number

3) Using typeof()

The typeof() returns the type of a variable. For a number, it returns number.

<script>
var a=50, b="Hello";
if(typeof(a)=="number"){
document.write(a + " (a) Is a number");
}
else{
document.write(a + " (a) Is not a number");
}

if(typeof(b)=="number"){
document.write(b + " (b) Is a number<br>")
}
else{
document.write(b + " (b) Is not a number<br>")
}
</script>
//Output
50 (a) Is a number
Hello (b) Is not a number