# NaN : Not A Number

NaN is known as *Not A Number*

It is a returned value

It is not configurable and not writable.

## Example of NaN

When we will get output as NaN ?
```
<script>
var a = 'xyz';
document.write(a*2); // Output NaN
</script>
```

## Not equal to itself

NaN is the only JavaScript value which is not equal to itself.
```
<script>
var a = NaN;
if(a === a){
document.write('Equal')
}else{
document.write('Not Equal')
}
</script>
```

The output will be *Not Equal*

To check the value is NaN, we can use isNaN →

User entered data from an input text box is always a string, so for any mathematical calculation we need to convert them to number by using parseFlot or parseInt. Otherwise we will get *NaN* as output of any mathematical calculation if we use directly these inputs.

String to Number by parseInt →

←JavaScript Math Reference

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Math Functions in JavaScript