Stop Using parseInt - Here's Why It's Slowing You Down

#javascript #performance #bestpractices #beginners

Have you ever used parseInt to truncate a number in JavaScript? I used to do it too. But today, I want to show you why that habit might be hurting your code - and how to do it better.

🧠 The Problem

parseInt was designed to convert strings to integers, not to truncate numbers. When you write:

parseInt(4.9); // 4

Does it work? Yes.
Is it clear? Not really.
Is it fast? Definitely not.

⚡️ The Better Way

If you're working with a number and want to remove the decimal part, use:

Math.trunc(4.9); // 4

It's faster, clearer, and semantically correct. It also works with negative numbers:

Math.trunc(-4.9); // -4

🧪 Performance Matters

In simple benchmarks, Math.trunc can be up to 5x faster than parseInt. That matters - especially in loops, animations, or real-time calculations.

🧨 Other Ways to Truncate Numbers in JavaScript (Use with Caution)

Sometimes you want to truncate a number without using Math.trunc(). Here are some alternatives - each with its own quirks.

🔸 Bitwise OR (| 0)

This is a clever trick to truncate decimals using bitwise operations.

4.9 | 0;    // 4
~~4.9;      // 4

✅ Fast
⚠️ Only works reliably for 32-bit integers. Avoid with large or precise numbers.

🔸 Double Bitwise NOT (~~)

Similar to | 0, but slightly more readable for some developers.

~~4.9;    // 4
~~-4.9;   // -4

✅ Fast
⚠️ Same limitations as | 0.

🔸 Math.floor() and Math.ceil()

These are rounding functions, not true truncation - but they can be useful depending on the sign of the number.

Math.floor(4.9);   // 4
Math.floor(-4.9);  // -5

Math.ceil(4.9);    // 5
Math.ceil(-4.9);   // -4

✅ Clear intent
⚠️ Not truncation - they round up/down.

🔸 Number.toFixed(0)

This method returns a string, not a number, and it rounds the value.

(4.9).toFixed(0);   // "5"
Number((4.9).toFixed(0)); // 5

✅ Useful for formatting
⚠️ Not truncation, and returns a string unless converted.

📊 Recommendation

Method	Truncates?	Returns	Performance	Notes
`Math.trunc()`	✅	Number	🔼 High	Best choice for clarity and safety
`parseInt()`	✅	Number	🔽 Low	Avoid for truncating numbers
`Math.floor()`	❌	Number	🔼 High	Rounds down, not truncates
`Math.ceil()`	❌	Number	🔼 High	Rounds up, not truncates
`toFixed(0)`	❌	String	🔽 Medium	Rounds and returns string unless converted

✅ Why Best Practices Matter

As a developer, you're not just writing for machines - you're writing for other humans, including your future self. Choosing the right tool makes your code:

Easier to read
Faster to run
More meaningful

💬 Final Thoughts

If you're truncating numbers with parseInt, stop now. Use Math.trunc and write code that makes sense. Small choices lead to better habits - and better software.

Enjoyed this tip? Share it with a friend who's still stuck in parseInt land. Let's do the basics right - then we can do the magic. 😉