StdFormat - A Powerful TypeScript/JavaScript String Formatter

GitHub Repo | NPM Pkg

About

StdFormat is a TypeScript/JavaScript library for powerful string formatting using { } notation, inspired by C++20 format and Python format.

Variants

There have been few separate npm packages of this project along the line. The recommended and main package is:

• std-format 👍 Use this!

Other two are kept up to date for legacy support, in case someone is using them:

• @sbrockma/std-format
• @tspro/brace-format

All three packages are equal and work what is written in this documentation.

IIFE Note: Global name StdFormat is declared by all three variants. @tspro/brace-format declares additional alias BraceFormat for legacy support.

Project Status

This project is now in maintenance. I will fix bugs and add features on request.

Install

npm i std-format

Usage

Import (ESM)

// Import default export
import StdFormat from "std-format";

StdFormat.format("{}, {}!", "Hello", "world");

// Or import named exports
import { format, int, float, setLocale, FormatError } from "std-format";

format("{}, {}!", "Hello", "world");

Require (CommonJS)

const StdFormat = require("std-format");

StdFormat.format("...");

Browser Script

Use the standalone IIFE bundle via unpkg or jsdelivr cdn.

<script src="https://unpkg.com/std-format@4.1.0/dist/index.global.js"></script>
<script src="https://cdn.jsdelivr.net/npm/std-format@4.1.0/dist/index.global.js"></script>

<script>
    const { format } = window.StdFormat;
    var str = format("{}, {}!", "Hello", "world");
</script>

API

format(str, ...args)

The main formatting function.

format("{}, {}!", "Hello", "world");

int() and float()

int() and float() are wrapper functions that can be used to force number to int or float.

format("{}", int(5));   // "5"
format("{}", float(5)); // "5.0"

Note: Formatting rules are strict.

format("{:.2e}", int(5)); // Throws, cannot format int as float.
format("{:d}", float(5)); // Throws, cannot format float as int.

float() simply wraps a number, while int() wraps a JSBI.BigInt, enabling support for large integers.

format("{:d}", int("111111111111111111111111111111"));

You can also pass BigInt to format(), it will be safely wrapped to int().

format("{:d}", BigInt("111111111111111111111111111111"));

setLocale(locale?)

Sets the locale for "n" and "L" specifiers:

setLocale("en-GB");
setLocale(); // Reset to default

FormatError

Thrown for formatting violations:

try {
    format("{:s}", 42);
} 
catch(e) {
    if(e instanceof FormatError) {
        console.error(e);
    }
}

Format Specification

StdFromat uses replacement fields like {}, {0}, {name}, with support for detailed format specs.

Replacement Field Structure

{field_id:arr_1:arr_2:arr_N:elem}

• field_id: field id/name
• arr_*: array/object presentations
• elem: element presentation

Any part can be empty string/omitted.

Element Presentation

Format specification for element:

[[fill]align][sign]["z"]["#"]["0"][width][grouping]["." precision]["L"][type]

Part	Description
fill	Any char
align	<, ^, >, =
sign	+, -, space
z	Force -0 to +0
#	Alternate form
0	Zero padding
width	Width or nested {}
grouping	, or _
precision	For floats/strings
L	Locale-aware
type	See type table below

Types:

Type	Description
Omitted	Default
s	String
c	Char (from int)
d	Decimal int
n	Decimal with locale
b / B	Binary
o	Octal
x / X	Hex
e / E	Scientific float
f / F	Fixed float
%	Percent
g / G	General float
a / A	Hex float

Array/Object Presentation

Format specification for array, set, map and object:

[[fill]align][width][type]

Part	Description
fill	Any char
align	<, ^, >
width	Width or nested {}
type	See type table below

Types:

Type	Output For Array/Set	Output For Map/Object
d / Omitted	[1, 2, 3]	[[a, 1], [b, 2], [c, 3]]
n	1, 2, 3	a: 1, b: 2, c: 3
b	{1, 2, 3}	{{a, 1}, {b, 2}, {c, 3}}
m		[a: 1, b: 2, c: 3]
s	123	a1b2c3

More Info

• More information about format specs: C++20 format and Python format.
• Formatting elements is quite standard as in the links above.
• Formatting arrays is less standard so array presentation used here is partly improvised.

Examples

// Using auto field numbering
format("{}{}", "A", "B"); // "AB"

// Using manual field numbering
format("{1}{0}", "A", "B"); // "BA"

// Using named fields
format("{name} {age:d}", { name: "Tim", age: 95 }); // "Tim 95"

// Fill, align and width
format("{:0<8d}", 777);  // "77700000"
format("{:0^8d}", 777);  // "00777000"
format("{:0>8d}", -777); // "0000-777"
format("{:0=8d}", -777); // "-0000777"

// Precision
format("{:.2f}", 1); // "1.00"

// String width
format("{:10.4s}", "Alligator");         // "Alli      "

// With nested arguments
format("{:{}.{}s}", "Alligator", 10, 4); // "Alli      "

// Array
format("{:d}", [1, 2, 3]); // "[1, 2, 3]"

// Set
format("{:d}", new Set([1, 2, 3, 2])); // "[1, 2, 3]"

// Map
format("{:m:}", new Map([["x", 1], ["y", -1]])); // "[x: 1.0, y: -1.0]"

// Object
format("{{{:n:}}}", { x: 1, y: -1}); // "{x: 1.0, y: -1.0}"

// Floating point types
format("{0:.3e} {0:.3f} {0:.3%} {0:.3g} {0:.3a}", Math.PI); // "3.142e+00 3.142 314.159% 3.14 1.922p+1"

// Integer types
format("{0:#b} {0:#o} {0:#d} {0:#x} {0:c}", 65); // "0b1000001 0o101 65 0x41 A"

Note!

Legacy JavaScript has only a single number type, not separate int and float.

// By default format number as float.
format("{}", 5);   // "5.0"

// To format number as integer, use type "d".
format("{:d}", 5); // "5"

// Or use int() and float() to force type.
format("{}", int(5));   // "5"
format("{}", float(5)); // "5.0"

Compatibility

This library is written in TypeScript and includes type declarations. Since v3.1.0, it is bundled into ESM, CJS, and IIFE formats using tsup (previously webpack). No changes should be required in the documented usage.

• Only ES5-compatible JavaScript functions are used.
• CJS and IIFE bundles are transpiled to ES5 for broad compatibility.
• The ESM bundle targets modern environments (ES6+).

While designed with compatibility in mind, the library has not been explicitly tested against specific Node.js or browser versions.

Report a Bug

Found a bug or unexpected behavior?

Please open a new issue.

You can also suggest a feature or impovement.

Thanks for helping improve the project!

License

This project is licensed under the MIT License.

It also bundles the JSBI library, which is licensed under the Apache License 2.0.