Indian Modassir

Posted on Aug 9 • Edited on Aug 12

JavaScript sprintf implementation

#node #printfy #javascript #webdev

🌟 Features

Fully functional printf, sprintf, and vsprintf methods.
C-style format specifiers support:
- %d, %f, %s, %x, %b, %o, %u, %c, %e, %g, %G, etc.
Padding, alignment, width, and precision controls
Argument indexing like %2$08.2f
Supports most common C-style format specifiers.
Distinct handling for %f (locale) and %F (non-locale)
Float, Integer, Hex, Octal, Binary, Char, and String formatting
Custom width, padding, alignment flags.
Float precision control up to JavaScript's max (53 bits).
Handles NaN, Infinity and signed values.
Works in Node.js, modern browsers, and CommonJS/AMD environments.
No external dependencies

How to install printfy

To include printfy in Node, first install with npm.

npm install printfy

How to build printfy

Clone a copy of the main Sizzle git repo by running:

git clone git://github.com/jsvibe/printfy.git

In the printfy/dist folder you will find build version of printfy along with the minified file.

Including printfy

Below are some of the most common ways to include printfy.

Browser

CDN Link

<script src="https://cdn.jsdelivr.net/npm/printfy@2.0.2/dist/printfy.min.js"></script>

You can add the script manually to your project:

<script src="printfy.js"></script>

Webpack / Browserify / Babel

There are several ways to use Webpack, Browserify or Babel. For more information on using these tools, please refer to the corresponding project's documentation. In the script, including printfy will usually look like this:

import {sprintf, printf, vprintf, vsprintf} from "printfy";

Node.js

Fully compatible with Node.js, this library lets you use familiar C-style printf formatting in server-side code—ideal for CLI tools, logging, and backend output formatting.

const {sprintf, printf, vprintf, vsprintf} = require("printfy");

API Usage

`printf(format, ...args)`

Logs the formatted output to the console.

printf("Hello %s, you have %d new messages", "Alice", 5);
// Output: Hello Alice, you have 5 new messages

`vprintf(format, args[])`

Logs the formatted output to the console.

printf("Hello %s, you have %d new messages", [ "Alice", 5]);
// Output: Hello Alice, you have 5 new messages

`sprintf(format, ...args)`

Returns a formatted string (like sprintf in C).

let result = sprintf("Value: %08.2f", 3.14);
console.log(result); // Output: Value: 00003.14

`vsprintf(format, args[])`

Same as sprintf, but accepts arguments as an array.

vsprintf("User: %s, Score: %d", ["Bob", 100]);
// Output: User: Bob, Score: 100

🔢 Supported Specifiers

Specifier	Meaning	Example
`%s`	String	`sprintf("%s", "abc")` → `"abc"`
`%S`	Uppercase string	`"abc"` → `"ABC"`
`%d`	Integer (decimal)	`42`
`%u`	Unsigned integer (64-bit)	`-1` → `"18446744073709551615"`
`%b`	Binary	`5` → `"101"`
`%o`	Octal	`8` → `"10"`
`%x`, `%X`	Hexadecimal (lower/upper)	`255` → `"ff"` or `"FF"`
`%f`, `%F`	Float (fixed-point)	`3.14` → `"3.140000"`
`%e`, `%E`	Float (scientific notation)	`3.14` → `"3.14e+0"`
`%g`, `%G`	Auto float/scientific (shortest form)	`123000` → `"1.23e+5"`
`%c`	Character from ASCII code	`65` → `"A"`
`%%`	Literal percent sign	`%`

⚙️ Format Options

Format syntax:

%[index$][padding][flag][width][.precision]specifier

🧪 Examples

sprintf("Binary: %08b", 5);          // Binary: 00000101
sprintf("Hex: %#x", 255);            // Hex: ff
sprintf("Char: %c", 65);             // Char: A
sprintf("Padded: %10s", "text");     // Padded:      text
sprintf("Left: %-10s!", "text");     // Left: text     !
sprintf("Float: %.2f", 3.14159);     // Float: 3.14
sprintf("Scientific: %.2e", 1200);   // Scientific: 1.20e+3

[index$] – Argument Indexing

sprintf('%2$s is %1$d years old.', 22, 'Modassir');
// Output: Modassir is 22 years old.

[padding] – Custom Padding Character

sprintf("%'~5d", 42);
// Output: ~~~42

sprintf("%'~-5d", 42);
// Output: 42~~~

[flag] – Format Flags

Support flags + and - only.

sprintf("%+d", 42);
// Output: +45

sprintf("%-d", 42);
// Output: -45

[width] – Custom Width

sprintf("%6s", "JS");
// Output: "    JS"

sprintf("lang%6s", "JS");
// Output: "lang    JS"

sprintf("%-6s", "JS");
// Output: "JS    "

sprintf("%-6slib", "JS");
// Output: "JS    lib"

[.precision] – Precision (floating-point or string truncation)

sprintf("%.2f", 3.14159);
// Output: 3.14

sprintf("%.4s", "OpenAI");
// Output: Open

Full Format

sprintf('%2$\'#-+10.2f and %1$\'*_10s', "JS", 3.14159);
// Output: +3.14#####JS*******

🔹 Explanation:

%2$: Second argument (3.14159)
'#: padding character #
-: left align
+: show sign
10: total width 10
.2f: 2 decimal places
%1$'*_10s: First argument (JS), padded with * to width 10

⚠️ Errors & Warnings

ArgumentCountError: Thrown if insufficient arguments are passed for format specifiers.
ValueError: Invalid specifier detected.
RangeError: Padding exceeds safe integer limit.
Precision Truncation: Float precision is capped at JavaScript's safe limit (53 digits).

🔄 Comparison with Other Libraries

Feature / Library	printfy ✓	sprintf-js 🟡	fast-printf 🟢	printf (npm) 🔵
C-style specifiers	✓ Full	✓ Full	✓ Partial	✓ Partial
%2\$ style arg index	✓ Yes	✓ Yes	✗ No	✗ No
BigInt support	✓ Yes	✗ No	✗ No	✗ No
%f vs %F (locale)	✓ Separate	✗ Combined	✗ Combined	✗ No
Custom padding ('x)	✓ `'x`, `0`, etc.	✗ No	✗ No	✗ No
String return (sprintf)	✓ Yes	✓ Yes	✓ Yes	✗ No
Lightweight / No deps	✓ Yes	✓ Yes	✓ Yes	✓ Yes
Unicode emoji safe	Partial	Partial	✗ No	✗ No
Performance	Good	Good	Best	Good