Using variable in setattribute function in javasscript

Remove the unary plus from +c

document.getElementById("ct").setAttribute('value',+c);

You are trying to type cast string (which doesnt represent a number) to numeric value prefixing it with + and hence seeing NaN (Not a Number), it should be:

document.getElementById("ct").setAttribute('value',c);

+ is a unary operator used for type casting to numeric value.

See Unary Plus operator

The unary plus operator precedes its operand and evaluates to its operand but attempts to converts it into a number, if it isn't already. For example, y = +x takes the value of x and assigns that to y; that is, if x were 3, y would get the value 3 and x would retain the value 3; but if x were the string "3", y would also get the value 3. Although unary negation (-) also can convert non-numbers, unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number. It can convert string representations of integers and floats, as well as the non-string values true, false, and null. Integers in both decimal and hexadecimal ("0x"-prefixed) formats are supported. Negative numbers are supported (though not for hex). If it cannot parse a particular value, it will evaluate to NaN.

Fiddle

Try this,

function submitcity(c){
   document.getElementById("ct").setAttribute('value',c);//remove + sign before c
   //document.cityform.submit();
   alert(c);
}