i'm a beginner on Javascript. I have a problem. When i use `parseInt()` then use `isNaN()`, it worked well with params `a,b,c` but with 
`d` it couldn't run into `if` condition to log the message.

Here is my code:

```js
let a = "abc";
let b = "123";
let c = "abc123";
let d = "123abc"; //problem
let check = parseInt(d); //b,c,d
if (isNaN(check)) {
  console.log('This is a invalid number!');
}
```

What is the simplest way to fix this?