The Mystery of Leading Zeros in JavaScript: A Fun Discovery

The Mystery of Leading Zeros in JavaScript: A Fun Discovery

One day, our intern, Mr. Shailendra, whom we call Lala Ji, found something strange. While working on a JavaScript project, he noticed that numbers with leading zeros were acting weird. This made me curious, so I decided to look into it more. What I found was interesting and taught me a lot about JavaScript's quirks and oddities.

Why does JavaScript interpret leading zeros differently?

A long time ago, in many programming languages like C, numbers with leading zeros were treated as octal (base 8) numbers. JavaScript followed this rule too. So, when a number starts with a zero (0), JavaScript thinks it’s an octal number.

Surprising Examples of Leading Zeros in JavaScript

Here’s what Lala Ji found:

let number = 010;
console.log(number); // Output: 8

You might think number would be 10, but it's actually 8! That’s because 010 is seen as an octal number, which is 8 in decimal (base 10).

Understanding Different Number Bases in JavaScript

JavaScript also understands other bases:

  • Hexadecimal (base 16): Starts with 0x or 0X.

  • Binary (base 2): Starts with 0b or 0B.

  • Octal (base 8): In modern JavaScript (ES6), use 0o or 0O.

For example:

let hex = 0x1A;    // Hexadecimal 1A -> Decimal 26
let binary = 0b1010; // Binary 1010 -> Decimal 10
let octal = 0o10;    // Octal 10 -> Decimal 8

How parseInt Handles Leading Zeros in JavaScript

The parseInt function turns strings into numbers. It can take a second argument called the radix, which tells it what base to use.

Without the radix, parseInt might confuse you:

let number1 = parseInt('010'); // Thinks it's octal -> 8
let number2 = parseInt('10');  // Thinks it's decimal -> 10

Always give the radix to avoid confusion:

let number1 = parseInt('010', 10); // Parse as decimal -> 10
let number2 = parseInt('010', 8);  // Parse as octal -> 8

Best Practices to Avoid Confusion with Leading Zeros in JavaScript

  1. Don't Use Leading Zeros: Don’t use leading zeros for decimal numbers. Use strings if you need to pad.

     let number = 10;       // Correct
     let paddedNumber = '010'; // Use a string for padding
  2. Use Clear Prefixes: Use 0o for octal, 0b for binary, and 0x for hexadecimal.

     let octal = 0o10;    // Clear octal
     let binary = 0b1010; // Clear binary
     let hex = 0x1A;      // Clear hexadecimal
  3. Use Strict Mode: Turn on strict mode to stop using old octal numbers.

     'use strict';
     let number = 010; // Error: Octal numbers not allowed in strict mode.
  4. Specify Radix withparseInt: Always give the radix when using parseInt.

     let decimal = parseInt('10', 10); // Clear decimal
     let octal = parseInt('10', 8);    // Clear octal


Thanks to Lala Ji, we learned something cool about JavaScript. Knowing these quirks helps us write better code. Always be curious and keep learning!

Happy coding!

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