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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