NodeJS string concatenation unusual behaviour - node.js

Question

I am trying to construct this string for printing one message.
"At position #[" + index + "][" + _subIndex + "] TLPHN_DVC_TYP " +
+ _telNum?.TelephoneDeviceType.toString() + " ,allowed " + telephoneDeviceTypeEnum.join(',');
from watch is VsCode:
where index =0;_subIndex =0;telNum.TelephoneDeviceType =Mobile;telephoneEnum=["Mobile","Landline"];
It's returning :
At position #[0][0] TLPHN_DVC_TYP NaN ,allowed Mobile,Landline
Full Code:
if (_telNum?.TelephoneDeviceType && !(telephoneDeviceTypeEnum.indexOf(_telNum.TelephoneDeviceType) > 0)){
console.log( "At position #[" + index + "][" + _subIndex + "] TLPHN_DVC_TYP " +
+ _telNum?.TelephoneDeviceType.toString() + " ,allowed " + telephoneDeviceTypeEnum.join(','));
}
the condition should not satisfy but not sure why it's going inside the if and NaN returning. any suggestion?

Answer 1

It's the two plus signs: "] TLPHN_DVC_TYP " + + _telNum?// ...etc. The second one is parsed as the unary +, or a conversion to number, which obviously fails. Compare:
console.log("foo" + "bar");
console.log("foo" + + "bar");
Added #1:
if (_telNum?.TelephoneDeviceType && !(telephoneDeviceTypeEnum.indexOf(_telNum.TelephoneDeviceType) >= 0)){
console.log( "At position #[" + index + "][" + _subIndex + "] TLPHN_DVC_TYP " +_telNum?.TelephoneDeviceType.toString() + " ,allowed " + telephoneDeviceTypeEnum.join(','));
}