Working with Big Numbers in JavaScript for Ethereum Applications

Ever tried moving money across an Ethereum app and suddenly noticed some weird decimal issues? Maybe you sent what looked like the right amount, but something seemed… off. That’s the reality of working with numbers in JavaScript for Ethereum.

Big numbers aren’t just large—they’re really large. And in blockchain, every single decimal matters. Tiny rounding errors could lead to financial disasters. The problem? JavaScript isn’t built to handle numbers at Ethereum’s scale natively. But there are ways around it.

Why JavaScript Struggles with Ethereum’s Big Numbers

JavaScript’s number type uses floating-point arithmetic, which is great for speed but terrible for precision. That’s a huge problem when dealing with cryptocurrency transactions, where even the smallest inaccuracy can cause real losses.

Ethereum Uses Huge Numbers—Bigger Than JavaScript Can Handle

Ethereum doesn’t just work with simple values like 10 or 0.5. It deals with wei, the smallest unit of Ether, and there are 10^18 wei in a single ETH. That means even a small transaction could involve numbers that JavaScript simply can’t represent accurately.

The Risk of Rounding Errors in Ethereum Transactions

A tiny rounding error might not sound like a big deal—until you’re working with financial transactions. Imagine an app that wrongly calculates how much gas to send. If it underestimates, the transaction fails. If it overestimates, the user loses money. Both are bad.

How to Handle Big Numbers in JavaScript

Since JavaScript’s built-in Number type isn’t reliable for Ethereum math, you’ll need something better.

Using BigInt for Integer Calculations

JavaScript introduced BigInt, which can handle large integers without losing precision. It’s perfect for Ethereum because most calculations don’t involve decimals—just huge whole numbers.

Example:

let largeNumber = BigInt("1000000000000000000"); // 1 ETH in wei
console.log(largeNumber * 2n); // 2 ETH in wei

BigInt works great for whole numbers, but Ethereum transactions sometimes involve decimals. That’s where specialized libraries come in.

The Power of bn.js and BigNumber.js

Most Ethereum applications rely on bn.js or BigNumber.js to handle precision safely.

• bn.js: Optimized for performance, used by Ethereum’s core libraries
• BigNumber.js: More user-friendly, great for financial calculations

Example using BigNumber.js:

const BigNumber = require('bignumber.js');
let ethValue = new BigNumber("1.5").times("1000000000000000000"); // Convert ETH to wei
console.log(ethValue.toString()); // "1500000000000000000"

With libraries like these, calculations stay precise, and you avoid JavaScript’s floating-point pitfalls.

Converting Between Units Safely

Ethereum applications constantly convert between units—ETH, Gwei, and wei. If done wrong, users might send way too much or too little.

Using Ether.js for Unit Conversion

Ether.js makes these conversions easy and accurate.

const { ethers } = require("ethers");
console.log(ethers.utils.parseEther("1.5").toString()); // "1500000000000000000" (1.5 ETH to wei)
console.log(ethers.utils.formatEther("1500000000000000000")); // "1.5" (wei to ETH)

No weird rounding errors. Just safe, reliable conversions.

Common Mistakes with Unit Conversion

• Using division instead of a library: Dividing by 10^18 can introduce floating-point issues.
• Forgetting wei is an integer: Don’t use decimals in wei values, or things will break.
• Relying on Number for big values: Even parseInt or toFixed() won’t save you from precision errors.

Handling Ethereum Transactions with Precision

Big numbers don’t just show up in calculations—they’re all over Ethereum smart contracts.

Reading and Writing Large Values from Contracts

When interacting with contracts, always use a BigNumber library to ensure correct values.

Example:

let contract = new ethers.Contract(contractAddress, contractABI, provider);
let balance = await contract.balanceOf(userAddress);
console.log(balance.toString()); // Always use .toString() to avoid unexpected behavior

Calculating Gas Fees Correctly

Gas fees fluctuate constantly, and getting them wrong can make transactions fail.

let gasPrice = await provider.getGasPrice();
console.log(ethers.utils.formatUnits(gasPrice, "gwei")); // Convert wei to Gwei safely

If you use Number(), you risk losing precision and paying the wrong amount. Always work with libraries that handle big numbers properly.

Storing and Displaying Big Numbers

Storing Ethereum-related numbers in databases or showing them in user interfaces requires extra care.

Converting for Storage

Many apps store big numbers as strings to prevent precision loss.

Example JSON storage:

{
  "userBalance": "123456789012345678901234567890"
}

This ensures numbers stay accurate, even if they’re huge.

Formatting Numbers for Display

Users don’t want to see huge, unreadable numbers. Formatting properly makes apps more user-friendly.

Example using ethers.js:

console.log(ethers.utils.commify("1000000000000000000")); // "1,000,000,000,000,000,000"

This adds commas so users can read values easily.

Dealing with External Data Sources

Fetching Ethereum-related numbers from APIs or smart contracts requires caution.

Avoiding JavaScript’s Number in API Responses

Many APIs return numbers as strings for a reason. Always handle them with a BigNumber library.

Bad approach:

let price = Number(apiResponse.price); // Might lose precision

Good approach:

let price = new BigNumber(apiResponse.price);

This keeps everything accurate, even when working with huge values.

Final Thoughts

Working with big numbers in JavaScript for Ethereum isn’t just about avoiding errors—it’s about making sure users don’t lose money. The wrong calculation could lead to failed transactions, wasted gas, or incorrect balances.

Using libraries like BigNumber.js, bn.js, and ethers.js ensures precision, keeps calculations accurate, and makes conversions safe. Whether you’re handling balances, gas fees, or smart contract interactions, always assume JavaScript’s default number system isn’t enough.

Ethereum demands precision. JavaScript isn’t perfect, but with the right tools, you can make sure your numbers are.