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Fazal Mansuri
Fazal Mansuri

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⚠️ JavaScript Precision Loss with Large Numbers - The silent bug that breaks applications without error

#javascript #webdev #frontend #backend

Imagine this scenario:

  • Backend API returns a perfectly correct number
  • Network tab shows the correct value
  • No errors, no warnings
  • But your frontend logic behaves incorrectly

Hours of debugging later…
You realize the number changed automatically 😨

Welcome to JavaScript precision loss - one of the most dangerous silent bugs in web development.

This blog explains:

  • ❓ Why this happens
  • 💥 How it silently breaks applications
  • 🔍 How to detect it
  • 🛠 How to fix it (frontend & backend)
  • 🧠 What both frontend and backend developers must know

🧠 The Root Cause: JavaScript Number Limit

JavaScript has only one numeric type:

Number
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Under the hood, it uses IEEE-754 double-precision floating-point format.

That means JavaScript can safely represent integers only up to:

Number.MAX_SAFE_INTEGER
// 9007199254740991
// which is 2^53 - 1
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Anything greater than this loses precision.

🚨 The Problem in Real APIs

Example API response (from backend): 

{
  "transactionId": 9007199254740993
}
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This value is valid, correct and often used for:

  • Database IDs
  • Transaction numbers
  • Order references
  • Snowflake IDs
  • Big integers from other systems

🔍 What You See in the Browser

🟢 Network → Response tab 

{
  "transactionId": 9007199254740993
}
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Looks perfect ✔️

🔴 Network → Preview tab (parsed by JS) 

{
  transactionId: 9007199254740992
}
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💥 Boom - value changed silently

No error
No warning
Just wrong data

😱 The Shocking Proof

Try this in the browser console:

9007199254740993 === 9007199254740992
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👉 Result:

true
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Yes. Two different numbers are considered equal.

That’s how dangerous this is.

🔥 Why This Is So Dangerous

This bug:

  • ❌ Does NOT throw errors
  • ❌ Does NOT break the UI immediately
  • ❌ Does NOT show warnings

But it can:

  • Break ID-based logic
  • Send wrong IDs in the next API call
  • Fail updates / deletes silently
  • Corrupt data flows
  • Cause production-only bugs

And debugging becomes a nightmare because:

“The backend sent the correct value.”

Which is true - but irrelevant.

🔄 Where It Breaks Exactly

The precision loss happens during:

JSON.parse()
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JavaScript parses JSON numbers into Number, and the precision is already lost at parse time.

By the time your code runs:

response.data.transactionId
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The damage is already done.

👨‍💻 Backend Developers: This Is Also Your Responsibility

If your API schema includes:

  • IDs
  • Counters
  • Financial values
  • Any number that may exceed 2^53 - 1

❌ Returning them as JSON numbers is unsafe.

✅ Best Backend Practice

Return large numeric identifiers as strings:

{
  "transactionId": "9007199254740993"
}
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This avoids precision loss entirely.

🧑‍💻 Frontend Developers: What If Backend Can’t Change?

Sometimes:

  • API is legacy
  • Schema cannot be changed
  • Third-party API returns big numbers

You still need a solution.

🛠 The Solution: json-bigint

json-bigint is a library that parses JSON without losing precision.

Install: 

npm install json-bigint
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Usage: 

import JSONbig from 'json-bigint';

const response = JSONbig.parse(apiResponseText);
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Now:

  • Large numbers are preserved
  • You can handle them safely
  • No silent corruption

You can choose:

  • BigInt
  • String representation
  • Custom handling logic

⚠️ Important Things to Remember

1️⃣ BigInt is NOT JSON-serializable

You must convert it before sending back to APIs.

2️⃣ Mixing Number & BigInt throws errors

Be consistent in comparisons and calculations.

3️⃣ IDs ≠ numbers

Treat IDs as identifiers, not math values.

🧪 How to Debug This Faster Next Time

If you suspect precision issues:

  1. Check Network → Response
  2. Compare with Network → Preview
  3. Log raw response text
  4. Compare values with strict equality
  5. Check against Number.MAX_SAFE_INTEGER

This can save hours of debugging.

🏁 Final Thoughts

This bug is dangerous because it:

  • Looks harmless
  • Produces no errors
  • Corrupts data silently

Once you know it, you’ll never forget it.

Key Takeaways:

  • JavaScript numbers have limits
  • APIs must respect client constraints
  • Large IDs should be strings
  • Silent bugs are the most expensive bugs

If this post helped you learn something new, you’re already ahead of many developers.

💬 Have you ever faced a bug where everything “looked correct” but wasn’t?
Drop your experience - let’s learn from each other.

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