JAVASCRIPT Data Type Issue

Question

I am currently learning JS and when I do some practice, I find some issues I am unclear on data type in Javascript. I understand that JS do NOT require specific type indication, it will automatically do the type conversion whenever possible. However, I suffer one problem when I do NOT do type conversion which is as follows:

 var sum = 0;
 function totalSum (a) {
 if (a == 0) {
 return sum;
 }
 else {
 sum += a;
 return totalSum(--a);
 }
 }
 var i = prompt("Give me an integer");
 // var num = parseInt(i); 
 alert("Total sum from 1 to " + i + " = " + totalSum(i));
 // alert("Total sum from 1 to " + i + " = " + totalSum(num));

I notice that the code works perfectly if I change the data type from string to int using parseInt function, just as the comment in the code does. BUT when I do NOT do the type conversion, things are getting strange, and I get a final result of 054321, if I input the prompt value as 5. AND in a similar way, input of 3, gets 0321 and so on.

Why is it the case? Can someone explain to me why the totalSum will be such a number? Isn't javascript will automatically helps me to turn it into integer, in order for it to work in the function, totalSum?

The sample code can also be viewed in http://jsfiddle.net/hphchan/66ghktd2/.

Thanks.

• Hacketo: i is a string, need to convert it to a number, Number(i)

Answer 1

I will try to decompose what's happening in the totalSum method.

First the method totalSum is called with a string as parameter, like doing totalSum("5");

Then sum += a; (sum = 0 + "5" : sum = "05") (note that sum become a string now)

then return totalSum(--a);, --a is converting the value of a to a number and decrement it's value. so like calling return totalSum(4);

Then sum += a (sum = "05" + 4 : sum = "054") ...

Answer 2

See the documentation of window.prompt: (emphasis mine)

result is a string containing the text entered by the user, or the value null.