When you write let x = 0123; in JavaScript, you might think you're assigning the number 123.
But JavaScript has a sneaky surprise in store… 😅
In this article, we’ll decode what happens when you use leading zeros in JavaScript numbers — and how it can trip up even experienced developers.
🔍 What Happens with let x = 0123;?
In non-strict mode, numbers that start with a 0 (zero) might be treated as octal (base 8).
let x = 0123;
console.log(x); // Output: 83
Why? Because:
0123 (octal) = 1×64 + 2×8 + 3×1 = 83
🤯 Yep, 0123 is NOT 123. It’s octal — which confuses the heck out of your logic if you didn’t mean it!
⚠️ Strict Mode Throws an Error
If you use "use strict", JavaScript disallows legacy octal literals:
"use strict";
let x = 0123; // ❌ SyntaxError: Octal literals are not allowed in strict mode.
✅ The Modern and Safe Way: Use 0o Prefix
To explicitly define octal numbers, use the ES6 octal literal syntax with a 0o prefix:
let x = 0o123;
console.log(x); // 83
✋ Common Mistakes to Watch Out For
❌ Using invalid digits in octal:
let y = 089; // SyntaxError in non-strict mode too
Octal only allows digits 0–7.
✅ Always define numeric literals intentionally:
let normal = 123; // Decimal
let octal = 0o123; // Octal (equals 83)
let hex = 0x1A; // Hexadecimal (equals 26)
let binary = 0b1010; // Binary (equals 10)
💡 Quick Quiz!
What’s the output of this code?
- let a = 010;
- console.log(a);
- A) 10
- B) 8
- C) SyntaxError
- D) 0o10
👉 Drop your answer in the comments!
🔚 Conclusion
Always be cautious when using leading zeros in JavaScript numbers — they might not mean what you think. Avoid legacy octal literals and use the modern 0o syntax when you really want base-8 values.
Have you ever been bitten by this quirk in production code? 😬 Let me know below!
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