How to set numeric value in environment variable

I got workaround. By using Number function convert string to integer.

So

if (postman.getEnvironmentVariable("mycounter") == 1 ) {
    postman.setEnvironmentVariable("result", "YES");
    postman.setEnvironmentVariable("mycounter", 1+Number(postman.getEnvironmentVariable("mycounter")));  
} else {
    postman.setEnvironmentVariable("result", "NO"); 
}

I think the sam's answer works but here is a cleaner way I use in one of my pre-request scripts

let myCounter = +environment["mycounter"];  // '+' Convert String into Integer
if (myCounter == 1) {
    myCounter++;
    postman.setEnvironmentVariable("result", "YES");
    postman.setEnvironmentVariable("mycounter", myCounter);  
} else {
    postman.setEnvironmentVariable("result", "NO"); 
}

More informations about converting ==> https://stackoverflow.com/a/1133814/1646479