NodeJS string concatenation unusual behaviour-CodePudding

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?

CodePudding user response：

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(','));
    }