JavaScript parseInt() bug

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There is a "bug" with the parseInt JavaScript function. The bug is not something that will affect you very often, but it is something you should be aware of. We've seen the bug in every browser except Opera.

I've created a button to demonstrate the bug. The bug is that parseInt can return an incorrect value. For example, parseInt("08") results in 0 instead of 8. And parseInt("09") results in 0 instead of 9. The reason for this is because the zero in front is trying to tell the browser that this is an octal (base 8) number, and "08" and "09" are not valid octal numbers. The button below builds statements from parseInt("01") through parseInt("09") and shows what the resulting value is. But it also does parseFloat("01") through parseFloat("09"). This shows that the bug does not exist with parseFloat.

Keep in mind that this bug only happens when the value being checked is a string and only when the string starts with a leading zero. So that's why it is difficult to notice. But if you're dealing with a web page that has user input, there's nothing prevening the user from entering 08 for a number field. To be 100% confident that you won't see the bug, use one of these two techniques:

parseInt(parseFloat(<my text value>))
 
 parseInt(<my text value>, 10)

The "10" in the second example tells the browser that base-10 values should be used. Here is the button so you can see what's happening. The button puts up an alert box with the result of the "buildString" function, which is shown below:

function buildString() {
    var ret = "";
    for (var i=1; i<=9; i++) {
       // Build a statement like parseInt("0?") where ? varies from "1" to "9"
       fn = "parseInt(\"0" + i + "\")";
       // Evaluate the statement to get the result
       ret += fn + " = " + eval(fn) + "\n";
       // Do the same thing, except with parseFloat instead of parseInt
       fn = "parseFloat(\"0" + i + "\")";
       ret += fn + " = " + eval(fn) + "\n";
       // This time do parseInt but specify base 10.
       fn = "parseInt(\"0" + i + "\", 10)";
       ret += fn + " = " + eval(fn) + "\n\n";
    }
    return ret;
 }

[yayak]

there are other techniques, try these:

parseInt(<my text value>.replace(/^0+/g, ''));
   ex.: parseInt("08".replace(/^0+/g, ''));
parseInt(<my text value> * 1);
   ex.: parseInt("08" * 1);

[pradeep]

Good, but the techniques mentioned above are technically correct.