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JavaScript

Posted by bumsu kim
bumsu kim
5,578 Points

parseInt does not work the way it is expected to.

I'd like to parse this hexadecimal number into decimal number. First line works fine, but the second line does not produce correct answer.

parseInt("0xA some random string", 16) ---> 10

parseInt("0xA", 16) ---> 16

Hexadecimal number 0xA is 10 in decimal number, so first line is correct but the second line is not. Could you explain?

bumsu kim
bumsu kim
5,578 Points

parseInt("0xA", 16) works, but it's parseInt(0xA, 16) that is not working. Since the function is supposed to convert the first argument into string and then parse it, it should work too.

1 Answer

Seth Shober
Seth Shober
30,240 Points
Seth Shober
Seth Shober
30,240 Points

Howdy!

Boy, is this a tough one. I feel like I've kind of been going in circles here, but think I can at least get you going in a good direction. I'm going to give a little extra clarity for other readers. Let's first take a look at exactly what parseInt(string, radix) is doing.

The parseInt() function parses a string argument and returns an integer of the specified radix.

string is the value to parse. If string is not a string, then it is converted to one. Leading whitespace in the string is ignored.

radix is an integer between 2 and 36 that represents the radix (the base in mathematical numeral systems).

Since the issue for you seems to be when not passing a string, I'm thinking something funky is happening with the coercion. Here's what I found out:

var num = 0xA
var str = '0xA'
num // 10
str // '0xA'
0xA // 10

//It looks like JS converted num to a decimal. Now we can compare.

parseInt(num, 16) // 16
parseInt(str, 16) // 10
parseInt(0xA) // 16
parseInt(10, 16) // 16
parseInt('10', 16) // 16

/* 
 * Either 0xA is being converted before parseInt is run,
 * or the parseInt toString call is converting it to a decimal, 
 * which is the default behavior seen here:
*/

// default
0xA.toString() // '10'
// passing a radix
0xA.toString(16) // 'a'

So, maybe the real question actually is: Is parseInt() the best option for what I am doing? To more directly answer you, the issue is because when you do parseInt(0xA, 16) it is really doing parseInt('10', 16).