filesize.js

A lightweight, high-performance file size utility for JavaScript that converts bytes to human-readable strings. Works in both Node.js and browser environments with comprehensive format support.

Installation

npm install filesize

Usage

ES Modules

import {filesize} from "filesize";
filesize(265318, {standard: "jedec"}); // "259.1 KB"

CommonJS

const {filesize} = require("filesize");
filesize(1024); // "1.02 kB"

Partial Application

import {partial} from "filesize";
const size = partial({standard: "jedec"});
size(265318); // "259.1 KB"

Parameters

• input {Number|String|BigInt} - The value to convert (required)
• options {Object} - Configuration object (optional)

Options Object

• base {Number} - Number base, default is 10
• bits {Boolean} - Enables bit sizes, default is false
• exponent {Number} - Specifies the symbol via exponent, e.g. 2 is MB for base 2, default is -1
• fullform {Boolean} - Enables full form of unit of measure, default is false
• fullforms {Array} - Array of full form overrides, default is []
• locale {String|Boolean} - BCP 47 language tag to specify a locale, or true to use default locale, default is ""
• localeOptions {Object} - Dictionary of options defined by ECMA-402 (Number.prototype.toLocaleString)
• output {String} - Output of function (array, exponent, object, or string), default is string
• pad {Boolean} - Decimal place end padding, default is false
• precision {Number} - Sets precision of numerical output, default is 0
• round {Number} - Decimal place, default is 2
• roundingMethod {String} - Rounding method, can be round, floor, or ceil, default is round
• separator {String} - Decimal separator character, default is an empty string
• spacer {String} - Character between the result and symbol, default is " "
• standard {String} - Standard unit of measure, can be iec, jedec, or si. Default is si (base 10)
• symbols {Object} - Dictionary of IEC/JEDEC symbols to replace for localization

Input Validation

The function validates input and throws TypeError for invalid values:

// Invalid input will throw TypeError
try {
  filesize("invalid");
} catch (error) {
  console.error(error.message); // "Invalid input"
}

try {
  filesize(NaN);
} catch (error) {
  console.error(error.message); // "Invalid input"
}

Testing

filesize.js maintains 100% test coverage across all metrics with a comprehensive test suite of 47 test cases:

-------------|---------|----------|---------|---------|-------------------
File         | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-------------|---------|----------|---------|---------|-------------------
All files    |     100 |      100 |     100 |     100 |                   
 filesize.js |     100 |      100 |     100 |     100 |                   
-------------|---------|----------|---------|---------|-------------------

Running Tests

# Run all tests (linting + unit tests)
npm test

# Run only unit tests
npm run mocha

Test Coverage

The test suite comprehensively covers:

• Basic functionality: Core conversion logic and edge cases
• Output formats: All output types (string, array, object, exponent)
• Standards support: IEC, JEDEC, and SI standards with different bases
• Bit conversion: Bits vs bytes with auto-increment logic
• Precision handling: Rounding methods and decimal precision
• Localization: Locale formatting and custom symbols
• Error handling: Invalid inputs and boundary conditions
• Partial functions: All option combinations with curried functions

Performance Benchmarks

filesize.js is optimized for high performance with comprehensive benchmarks covering various usage patterns:

🚀 Performance Overview

Scenario	Operations/sec	Notes
Basic conversion	~16-27M ops/sec	Fastest operations (large numbers)
Small numbers	~18-20M ops/sec	Consistent performance
With options	~5-13M ops/sec	Depends on option complexity
Locale formatting	~91K ops/sec	Most expensive operation
Stress testing	~2-10M ops/sec	Handles edge cases gracefully

📊 Detailed Benchmark Results

Basic Performance (5-run average)

• filesize(0): 18.6M ops/sec
• filesize(512): 20.3M ops/sec
• filesize(1024): 18.7M ops/sec
• filesize(1048576): 23.5M ops/sec
• filesize(1073741824): 23.6M ops/sec
• filesize(1099511627776): 26.9M ops/sec
• With bits=true: 16.8M ops/sec
• With standard="iec": 16.6M ops/sec
• With round=4: 13.4M ops/sec

Options Performance Impact

• bits=true: 12.5M ops/sec
• pad=true: 5.6M ops/sec
• locale="en-US": 91K ops/sec (significant overhead)
• standard="iec": 8.8M ops/sec
• standard="jedec": 9.0M ops/sec
• output="array": 10.2M ops/sec
• output="object": 9.2M ops/sec
• fullform=true: 7.8M ops/sec
• precision=3: 6.3M ops/sec
• separator=",": 7.2M ops/sec

Stress Test Results

• Edge cases: 2.3M ops/sec (90% success rate)
• Very large numbers: 4.6M ops/sec (100% success)
• Very small numbers: 10.4M ops/sec (100% success)
• Negative numbers: 5.4M ops/sec (100% success)
• Random options: 2.3M ops/sec (100% success)
• BigInt values: 3.7M ops/sec (100% success)
• Memory pressure: 49K ops/sec (100% success)
• Error conditions: 715K ops/sec (~40% success rate)

Partial Function Performance

Partial functions maintain excellent performance with minimal overhead:

• Acceptable overhead: 1.1-1.4x slower for most configurations
• Locale partials: Significant overhead (~180x slower) due to locale formatting
• Creation cost: Amortized across multiple uses

💡 Performance Insights

Excellent Performance (>10M ops/sec)

• Basic conversions with minimal options
• Large number processing (1TB+ values)
• Standard output formats (string, array, object)
• IEC and JEDEC standards

Good Performance (1-10M ops/sec)

• Complex option combinations
• Precision and rounding operations
• Fullform output
• Stress test scenarios

Use Sparingly (<100K ops/sec)

• Locale formatting (significant overhead ~91K ops/sec)
• Memory pressure conditions

🎯 Optimization Tips

1. Cache partial functions for repeated operations with same options
2. Avoid locale formatting in performance-critical code
3. Use object output for fastest structured data
4. Batch similar operations together
5. Profile your specific usage patterns

Running Benchmarks

# Run all benchmarks
cd benchmarks && node index.js

# Run specific benchmark
node benchmarks/basic-performance.js

# With garbage collection (more accurate)
node --expose-gc benchmarks/index.js

🔥 Recent Performance Optimizations (v11.0.8)

The latest version includes significant performance improvements:

• Pre-computed lookup tables for Math operations (eliminates expensive Math.pow() calls)
• Optimized base/standard logic with reduced branching
• Fast path for zero values with minimal computation
• Cached object property access to reduce repeated lookups
• Improved mathematical operations with conditional calculations

Overall performance improvement: 30-70% faster across common use cases while maintaining full backward compatibility.

Benchmarks run on macOS ARM64, Node.js v24.8.0, 12 CPU cores, 24GB RAM (5-run averages)

API Reference

Functions

filesize(input, options)

Converts a numeric value to a human-readable file size string.

Parameters:

• input {Number|String|BigInt} - The value to convert
• options {Object} - Configuration options (optional)

Returns: {String|Array|Object|Number} - Formatted size based on output option

filesize(500); // "500 B"
filesize(1024, {base: 2}); // "1 KiB"
filesize(265318, {output: "array"}); // [265.32, "kB"]

See also: partial()

partial(options)

Creates a pre-configured filesize function with options applied.

Parameters:

• options {Object} - Configuration options to apply

Returns: {Function} - New function with options pre-applied

const formatBinary = partial({base: 2, standard: "iec"});
formatBinary(1048576); // "1 MiB"

const formatBits = partial({bits: true});
formatBits(1024); // "8.19 kbit"

See also: filesize()

Output Formats

String Output (default)

filesize(265318); // "265.32 kB"
filesize(265318, {separator: ","}); // "265,32 kB"

Array Output

filesize(265318, {output: "array"}); // [265.32, "kB"]
filesize(1024, {output: "array", base: 2}); // [1, "KiB"]

Object Output

filesize(265318, {output: "object"}); 
// {value: 265.32, symbol: "kB", exponent: 1, unit: "kB"}

Exponent Output

filesize(1024, {output: "exponent"}); // 1
filesize(1048576, {output: "exponent", base: 2}); // 2

Standards Support

SI (International System of Units) - Default

filesize(1000); // "1 kB" (base 10)
filesize(1000000); // "1 MB"

IEC (International Electrotechnical Commission)

filesize(1024, {standard: "iec", base: 2}); // "1 KiB"
filesize(1048576, {standard: "iec", base: 2}); // "1 MiB"

JEDEC (Joint Electron Device Engineering Council)

filesize(1024, {standard: "jedec"}); // "1 KB"
filesize(1048576, {standard: "jedec"}); // "1 MB"

Examples

Basic Usage

import {filesize} from "filesize";

filesize(500); // "500 B"
filesize(1024); // "1.02 kB"
filesize(265318); // "265.32 kB"
filesize(265318, {round: 0}); // "265 kB"

Binary Formats

// IEC binary prefixes (KiB, MiB, GiB)
filesize(1024, {base: 2, standard: "iec"}); // "1 KiB"
filesize(1048576, {base: 2, standard: "iec"}); // "1 MiB"

// JEDEC binary format (KB, MB, GB with binary calculation)
filesize(1024, {standard: "jedec"}); // "1 KB"
filesize(265318, {standard: "jedec"}); // "259.1 KB"

Bits vs Bytes

filesize(500, {bits: true}); // "4 kbit"
filesize(1024, {bits: true}); // "8.19 kbit"
filesize(1024, {bits: true, base: 2}); // "8 Kibit"

Custom Formatting

// Full form units
filesize(1024, {fullform: true}); // "1.02 kilobytes"
filesize(1024, {base: 2, fullform: true}); // "1 kibibyte"

// Custom separators and spacing
filesize(265318, {separator: ","}); // "265,32 kB"
filesize(265318, {spacer: ""}); // "265.32kB"

// Precision and padding
filesize(1536, {round: 3, pad: true}); // "1.536 kB"
filesize(1536, {precision: 3}); // "1.54 kB"

Localization

// German locale
filesize(265318, {locale: "de"}); // "265,32 kB"

// Custom symbols
filesize(1, {symbols: {B: "Б"}}); // "1 Б"

// Custom full forms
filesize(12, {fullform: true, fullforms: ["байтов"]}); // "12 байтов"

Advanced Usage

// Specific exponent
filesize(1024, {exponent: 0}); // "1024 B"
filesize(1024, {exponent: 1}); // "1.02 kB"

// BigInt support
filesize(BigInt(1024), {standard: "jedec"}); // "1 KB"

// Extreme precision for very large numbers
filesize(Math.pow(1024, 8), {precision: 3}); // "1.21 YB"

Partial Application Patterns

import {partial} from "filesize";

// Create specialized formatters
const formatBinary = partial({base: 2, standard: "iec"});
const formatBits = partial({bits: true});
const formatPrecise = partial({round: 3, pad: true});
const formatGerman = partial({locale: "de"});

// Use throughout application
formatBinary(1048576); // "1 MiB"
formatBits(1024); // "8.19 kbit"
formatPrecise(1536); // "1.536 kB"
formatGerman(265318); // "265,32 kB"

// Method chaining equivalent
const sizes = [1024, 2048, 4096];
sizes.map(formatBinary); // ["1 KiB", "2 KiB", "4 KiB"]

Development

This project follows Node.js best practices and uses:

• ES Modules for modern JavaScript
• Mocha for testing with comprehensive coverage
• ESLint for code quality and consistency
• Rollup for building distributions
• TypeScript definitions for type safety

Project Structure

filesize.js/
├── src/
│   ├── filesize.js      # Main implementation
│   └── constants.js     # Unit definitions and constants
├── tests/
│   └── unit/
│       └── filesize.test.js  # Comprehensive test suite
├── types/
│   ├── filesize.d.ts    # TypeScript definitions
│   └── constants.d.ts   # Constants type definitions
└── package.json         # Dependencies and scripts

Contributing

1. Fork the repository
2. Create your feature branch (git checkout -b feature/amazing-feature)
3. Write tests for your changes
4. Ensure all tests pass (npm test)
5. Commit your changes (git commit -m 'Add amazing feature')
6. Push to the branch (git push origin feature/amazing-feature)
7. Open a Pull Request

Development Workflow

filesize.js includes an optimized development workflow with modern build tools:

• 🔄 Live Reload: Use npm run dev for automatic rebuilds during development
• 📊 Bundle Analysis: Monitor build sizes with npm run build:analyze
• ⚡ Fast Testing: Live test running with npm run test:watch
• 🔧 Auto-fixing: Automatic linting fixes with npm run lint:fix
• 📈 Performance: Optimized Rollup configuration with enhanced tree shaking

Build Output Analysis:

• Minified ES Module: ~1.8KB (gzipped)
• UMD Bundle: ~1.9KB (gzipped)
• Comprehensive source maps included

Development Commands

# Install dependencies
npm install

# Development mode with live rebuild
npm run dev

# Build distribution
npm run build

# Build with bundle size analysis
npm run build:analyze

# Live rebuild during development
npm run build:watch

# Run linting
npm run lint

# Auto-fix linting issues
npm run lint:fix

# Run tests
npm test

# Live testing during development
npm run test:watch

# Run only unit tests
npm run mocha

# Bundle size analysis
npm run analyze:size

# Benchmarking
npm run benchmark
npm run benchmark:basic
npm run benchmark:stress

License

Copyright (c) 2025 Jason Mulligan
Licensed under the BSD-3 license.