javascript

Why parseInt(’09’) Returns 0

If you’ve ever encountered the puzzling behavior of parseInt('09') returning 0 in JavaScript, you’re not alone. This quirk stems from how JavaScript’s parseInt() function handles numeric strings with leading zeros. Meanwhile, if you’re a developer or entrepreneur looking for where to build mobile apps in Abuja, Nigeria, See some of the projects we’ve worked on and contact us here.

Why Does `parseInt('09')` Return 0?

1. How `parseInt()` Works

JavaScript’s parseInt() converts a string to an integer, but it has a few unexpected behaviors:

If the string starts with 0x or 0X, it’s parsed as hexadecimal (base 16).
If the string starts with 0 (but not 0x), older JavaScript engines (pre-ES5) treated it as octal (base 8).
Modern JavaScript (ES5+) defaults to decimal (base 10), but some environments still follow legacy behavior.

2. The Problem with `parseInt('09')`

In older JS engines, 09 was interpreted as an octal number, but 9 is invalid in octal (digits must be 0-7).
Since 9 isn’t a valid octal digit, parseInt() stops parsing and returns 0.

3. The Fix: Always Specify a Radix

To avoid ambiguity, always pass the radix (base) as the second argument:

parseInt('09', 10); // Correct: returns 9  
parseInt('0x10', 16); // Correct: returns 16

4. Alternatives to `parseInt()`

Number() – Converts strings directly to numbers (doesn’t handle hex/octal ambiguity).

  Number('09'); // Returns 9

+ Operator – A shorthand for Number().

  +'09'; // Returns 9

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Conclusion

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