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JavaScript sprintf implementation

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Package description

What is sprintf-js?

The sprintf-js package is a JavaScript implementation of the sprintf function, which is originally from the C programming language. It provides string formatting capabilities, allowing users to format strings with placeholders that are replaced by specified values in a controlled manner.

What are sprintf-js's main functionalities?


The sprintf function allows you to format a string with placeholders, such as %s for strings, %d for integers, and many others. The placeholders are replaced by the provided arguments in order.

const sprintf = require('sprintf-js').sprintf;
const formattedString = sprintf('Hello, %s!', 'World');


The vsprintf function is similar to sprintf but takes an array of arguments instead of a variable number of arguments, which can be useful when the number of values to substitute is dynamic or not known in advance.

const vsprintf = require('sprintf-js').vsprintf;
const formattedString = vsprintf('There are %d %s', [3, 'apples']);

Other packages similar to sprintf-js




  • fix LICENSE name




Build Status NPM Version Dependency Status devDependency Status

sprintf-js is a complete open source JavaScript sprintf implementation for the browser and Node.js.

Note: as of v1.1.1 you might need some polyfills for older environments. See Support section below.


var sprintf = require('sprintf-js').sprintf,
    vsprintf = require('sprintf-js').vsprintf

sprintf('%2$s %3$s a %1$s', 'cracker', 'Polly', 'wants')
vsprintf('The first 4 letters of the english alphabet are: %s, %s, %s and %s', ['a', 'b', 'c', 'd'])



npm install sprintf-js


bower install sprintf



Returns a formatted string:

string sprintf(string format, mixed arg1?, mixed arg2?, ...)


Same as sprintf except it takes an array of arguments, rather than a variable number of arguments:

string vsprintf(string format, array arguments?)

Format specification

The placeholders in the format string are marked by % and are followed by one or more of these elements, in this order:

  • An optional number followed by a $ sign that selects which argument index to use for the value. If not specified, arguments will be placed in the same order as the placeholders in the input string.
  • An optional + sign that forces to precede the result with a plus or minus sign on numeric values. By default, only the - sign is used on negative numbers.
  • An optional padding specifier that says what character to use for padding (if specified). Possible values are 0 or any other character preceded by a ' (single quote). The default is to pad with spaces.
  • An optional - sign, that causes sprintf to left-align the result of this placeholder. The default is to right-align the result.
  • An optional number, that says how many characters the result should have. If the value to be returned is shorter than this number, the result will be padded. When used with the j (JSON) type specifier, the padding length specifies the tab size used for indentation.
  • An optional precision modifier, consisting of a . (dot) followed by a number, that says how many digits should be displayed for floating point numbers. When used with the g type specifier, it specifies the number of significant digits. When used on a string, it causes the result to be truncated.
  • A type specifier that can be any of:
    • % — yields a literal % character
    • b — yields an integer as a binary number
    • c — yields an integer as the character with that ASCII value
    • d or i — yields an integer as a signed decimal number
    • e — yields a float using scientific notation
    • u — yields an integer as an unsigned decimal number
    • f — yields a float as is; see notes on precision above
    • g — yields a float as is; see notes on precision above
    • o — yields an integer as an octal number
    • s — yields a string as is
    • t — yields true or false
    • T — yields the type of the argument1
    • v — yields the primitive value of the specified argument
    • x — yields an integer as a hexadecimal number (lower-case)
    • X — yields an integer as a hexadecimal number (upper-case)
    • j — yields a JavaScript object or array as a JSON encoded string


Argument swapping

You can also swap the arguments. That is, the order of the placeholders doesn't have to match the order of the arguments. You can do that by simply indicating in the format string which arguments the placeholders refer to:

sprintf('%2$s %3$s a %1$s', 'cracker', 'Polly', 'wants')

And, of course, you can repeat the placeholders without having to increase the number of arguments.

Named arguments

Format strings may contain replacement fields rather than positional placeholders. Instead of referring to a certain argument, you can now refer to a certain key within an object. Replacement fields are surrounded by rounded parentheses - ( and ) - and begin with a keyword that refers to a key:

var user = {
    name: 'Dolly',
sprintf('Hello %(name)s', user) // Hello Dolly

Keywords in replacement fields can be optionally followed by any number of keywords or indexes:

var users = [
    {name: 'Dolly'},
    {name: 'Molly'},
    {name: 'Polly'},
sprintf('Hello %(users[0].name)s, %(users[1].name)s and %(users[2].name)s', {users: users}) // Hello Dolly, Molly and Polly

Note: mixing positional and named placeholders is not (yet) supported

Computed values

You can pass in a function as a dynamic value and it will be invoked (with no arguments) in order to compute the value on the fly.

sprintf('Current date and time: %s', function() { return new Date().toString() })


You can use sprintf and vsprintf (also aliased as fmt and vfmt respectively) in your AngularJS projects. See demo/.



sprintf-js runs in all active Node versions (4.x+).


sprintf-js should work in all modern browsers. As of v1.1.1, you might need polyfills for the following:

  • String.prototype.repeat() (any IE)
  • Array.isArray() (IE < 9)
  • Object.create() (IE < 9)



sprintf-js is licensed under the terms of the BSD 3-Clause License.


1 sprintf doesn't use the typeof operator. As such, the value null is a null, an array is an array (not an object), a date value is a date etc.


Package last updated on 11 Sep 2023

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