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0.6.1 • Public • Published

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A complete implementation of the printf C functions family for Node.JS, written in pure JavaScript.

Bonus! You get extra features, like the %O converter (which inspects the argument). See Extra Features below.


Via NPM:

$ npm install printf


Use it like you would in C (printf/sprintf):

var printf = require('printf');
var result = printf(format, args...);

It can also output the result for you, as fprintf:

var printf = require('printf');
printf(write_stream, format, args...);


Flag   (space)

assert.eql('  -42', printf('% 5d', -42));

Flag + (plus)

assert.eql('  +42', printf('%+5d', 42));

Flag 0 (zero)

assert.eql('00042', printf('%05d', 42));

Flag - (minus)

assert.eql('42   ', printf('%-5d', 42));

Width / precision

assert.eql('42.90', printf('%.2f', 42.8952));
assert.eql('042.90', printf('%06.2f', 42.8952));

Numerical bases

assert.eql('\x7f', printf('%c', 0x7f));
assert.eql('a', printf('%c', 'a'));
assert.eql('"', printf('%c', 34));


assert.eql('10%', printf('%d%%', 10));
assert.eql('+hello+', printf('+%s+', 'hello'));
assert.eql("a", printf("%c", "a"));
assert.eql('"', printf("%c", 34));
assert.eql('$', printf('%c', 36));
assert.eql("10", printf("%d", 10));

Extra features!


The %O converter will call util.inspect(...) at the argument:

assert.eql("Debug: { hello: 'Node', repeat: false }",
  printf('Debug: %O', {hello: 'Node', "repeat": false})
assert.eql("Test: { hello: 'Node' }",
  printf('%2$s: %1$O', {"hello": 'Node'}, 'Test')

Important: it's a capital "O", not a zero!

Specifying a precision lets you control the depth up to which the object is formatted:

assert.eql("Debug: { depth0: { depth1_: 0, depth1: [Object] } }",
  printf('Debug: %.1O', {depth0: {depth1: {depth2: {depth3: true}}, depth1_: 0}})

You can use the alternative form flag together with %O to disable representation of non-enumerable properties (useful for arrays):

assert.eql("With non-enumerable properties: [ 1, 2, 3, 4, 5, [length]: 5 ]",
  printf('With non-enumerable properties: %O', [1, 2, 3, 4, 5])
assert.eql("Without non-enumerable properties: [ 1, 2, 3, 4, 5 ]",
  printf('Without non-enumerable properties: %#O', [1, 2, 3, 4, 5])

You can use the sign flag together with %O to enable colors in util.inspect:

assert.eql("With colors: { bar: \u001b[33mtrue\u001b[39m, baz: \u001b[33mfalse\u001b[39m }",
  printf('With colors: %+O', {bar: true, baz: false})

Argument mapping

In addition to the old-fashioned n$, you can use hashes and property names!

assert.eql('Hot Pockets',
  printf('%(temperature)s %(crevace)ss', {
    temperature: 'Hot',
    crevace: 'Pocket'
assert.eql('Hot Pockets',
  printf('%2$s %1$ss', 'Pocket', 'Hot')


Length and precision can now be variable:

assert.eql(' foo', printf('%*s', 'foo', 4));
assert.eql('      3.14', printf('%*.*f', 3.14159265, 10, 2));
assert.eql('000003.142', printf('%0*.*f', 3.14159265, 10, 3));
assert.eql('3.1416    ', printf('%-*.*f', 3.14159265, 10, 4));


Tests are written in CoffeeScript and are executed with Mocha. To use it, simple run npm install, it will install Mocha and its dependencies in your project's node_modules directory followed by npm test.

To run the tests:

npm install
npm test

The test suite is run online with Travis against the versions 6, 7, 8 and 9 of Node.js.


This package is developed by Adaltas.

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  • david