Fast printf-style string formatting with custom formatters and CSS styling support

@visulima/fmt

Fast, flexible string formatting utility inspired by Node.js's util.format. Format strings with printf-style placeholders, add custom formatters, and even apply CSS-like styling to terminal output—all with better performance than the standard library.

Key Features

printf-Style Formatting

Familiar %s, %d, %i, %f placeholders
JSON serialization with %j
Object inspection with %o and %O
Automatic type conversion

CSS Styling Support

Apply terminal colors with CSS syntax
%c placeholder for inline styles
Supports color, background, text decoration

Customizable

Build custom format functions
Add your own format specifiers
Custom stringify functions
Fast and optimized

Quick Start

Install the package:

npm install @visulima/fmt

Format a string:

import { format } from "@visulima/fmt";

const result = format("Hello %s, you have %d new messages", ["Alice", 5]);
console.log(result); // "Hello Alice, you have 5 new messages"

Use Cases

Format Log Messages

Create structured log messages with dynamic values:

import { format } from "@visulima/fmt";

function log(level: string, message: string, ...args: unknown[]) {
  const formatted = format(message, args);
  console.log(`[${level}] ${formatted}`);
}

log("INFO", "User %s logged in from %s", "alice", "192.168.1.1");
// [INFO] User alice logged in from 192.168.1.1

log("ERROR", "Failed to connect to database: %j", { host: "localhost", port: 5432 });
// [ERROR] Failed to connect to database: {"host":"localhost","port":5432}

Styled Terminal Output

Add colors and styles using CSS syntax:

import { format } from "@visulima/fmt";

const styled = format(
  "%cSuccess!%c Operation completed in %dms",
  ["color: green; font-weight: bold", "", 127]
);

console.log(styled);
// Outputs "Success!" in bold green, then normal text

Custom Date Formatter

Build a formatter with custom placeholders:

import { build } from "@visulima/fmt";

const format = build({
  formatters: {
    t: (time) => new Date(time).toLocaleString(),
    T: (time) => new Date(time).toISOString(),
  }
});

const message = format("Event occurred at %t (ISO: %T)", [Date.now(), Date.now()]);
console.log(message);
// Event occurred at 2/16/2026, 12:30:00 PM (ISO: 2026-02-16T11:30:00.000Z)

Next Steps

Installation

Learn how to install and import the package

Usage Guide

Explore all format specifiers and options

API Reference

Complete API documentation

Browser and Server Support

This package works in both Node.js and browser environments:

Node.js: ≥22.13 ≤25.x (full support)
Browsers: Compatible with ES6+ browsers
Deno/Bun: Fully compatible

Note: CSS styling (%c) only works in environments with ANSI color support.

Introduction