TypeError: .substring is not a function

问题

I'm running Acrobat Pro DC. I have a text field ("CC.CardNumber") with an Action/OnBlur event that runs the following script that produces this error. I've researched this for hours and can't see where my code is incorrect.

var fld = this.getField("CC.CardNumber");    
var first = fld.value.substring(0,1);    
fld.value = first; 

TypeError: fld.value.substring is not a function 3:AcroForm:CC.CardNumber:Annot1:OnBlur:Action1Exception in line 1051 of function AFSimple_Calculate, script byteCodeTool

回答1:

if it is a card number then you need to convert it into a String.

var first = fld.value.toString().substring(0,1);

来源：https://stackoverflow.com/questions/48013280/typeerror-substring-is-not-a-function